VOPlot – The VOTable plotting utility (Version 1.1) |  |  |

Introduction. 1

Getting Started. 1

Plotting Scatter Plots. 3

Plotting Histograms. 4

Zooming. 5

Select Points. 5

Unselect Points. 6

Clear All Selections. 11

Overlaying Plots. 14

Overlaying Histograms. 38

Transformations on columns. 53

Creating data subsets by applying filters. 91

Statistical Functions on Plotted Data. 119

Display VOTable. 144

Display Data. 175

Saving plot as an image. 200

Printing plot 204

## Introduction

VOPlot (VOTable Plotting tool) is an applet for plotting different astronomical graphs using data stored in VOTable format. VOPlot is available in stand-alone version and a web-based version. The web-based version is integrated with the VizieR Catalogue Service and can be used to plot any catalogue by selecting output layout as "Plot (VOPlot)". The stand-alone version can be downloaded from http://vo.iucaa.ernet.in/~voi/voplot.htm.

Click here for Release Notes and disclaimer information.

About VOPlot

VOPlot has been developed as a part of the Virtual Observatory - India initiative by Persistent Systems and the Inter-University Centre for Astronomy and Astrophysics (IUCAA), in collaboration with Centre de Données astronomiques de Strasbourg (CDS), with a support from the European AVO project. The collaboration between VO-I and CDS extends to several related projects.

VOPlot uses Ptplot 5.2, a 2D data plotter and histogram tool implemented in Java. Ptplot has been developed at EECS department at the University of California, Berkeley.

###### Differences in Stand-alone and Web-based VOPlot

The stand-alone version has same functionality as the web-based version except for a feature for saving the plot as an EPS file or as a VOTable file. Hence the

the save button in the stand-alone version is replaced by the help button in the web-based one. In the web-based version, there are no buttons to Browse and load a VOTable.

## Getting Started

Standalone Version:

To use the standalone version of VOPlot, you will need to download the executable jar file named voplot.jar. The file can be executed by typing the following command at the command prompt:

java -jar voplot.jar

This will open a java application window as shown in the Fig. 0. To use data from a VOTable you will need to load it first. You can either specify the URL of a VOTable or specify the path of a VOTable on your personal computer (PC) by using the “Browse” button. The name of the selected VOTable will appear in the text box. Click on the “Load” button to start using the VOTable. The message on the status bar will indicate whether the file was loaded successfully or not.

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#### Figure 0

Click to see larger image.

Web Version:

##### To use the Web-based version of VOPlot for plotting a VOTable:

1. Search for the required catalogue on VizieR, select a VOTable from the list and select the output layout as “Plot (VOPlot)”.

2. After clicking on "Submit Query" button the plotting applet loads. Note: Only numeric columns from the VOTables can be used for plotting.

3. Once the applet is loaded, you can see the numeric fields listed as columns in the dropdown list box.

#### Figure 1

Click to see larger image.

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## Plotting Scatter Plots

To draw a plot of one columns against the other,

1. Select the column to be plotted on the X-axis.
2. Select the column to be plotted on the Y-axis.
3. Click "Plot" button.

You can see the scatter plot. If required, you can plot data points on a log scale by setting the option appropriately using the "Log" checkbox.

#### Figure 2

Click to see larger image.

You can draw connected line plots by checking the “Connect” checkbox in the Plot Format dialog box. To change various plot properties, such as title, labels ranges etc., click on the format icon .

Figure 3

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## Plotting Histograms

To draw the histogram of a column

1. Select the column on X-axis.
2. Click "Histogram" button.

A sample histogram is shown in Figure 4.

Figure 4

Click to see larger image.

To plot the histogram bar heights on a logarithmic scale, check the "Log" checkbox corresponding to the Y-axis. If you want to plot the histogram of data points on a log scale, then check the "Log" checkbox corresponding to the X-axis.

When drawing histograms on a logarithmic Y-axis, the bars with height 1 cannot be drawn. You can force the VOPlot to draw such bars by incrementing the height of all the bars by 1. This is done by checking the “Incremented Y” checkbox in the Histogram Properties dialog box

To change the histogram properties, including the bin width, click on the format icon .

Figure 5

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## Zooming

For zooming into the plot you need to be in the Zoom mode. To go in zoom mode, click on the "Zoom" icon represented by

The "Zoom" icon represented by in depressed mode indicates that the mouse is currently in Zoom mode.

To zoom in, drag the left mouse button down and to the right to draw a box around an area that you want to see in detail. To zoom out, drag the left mouse button up and to the left. To get the original graph click on the "Reset" icon represented by .

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## Select Points

To take the mouse into select mode, click on the "Select" icon represented by

The "Select" icon represented by in depressed mode indicates that the mouse is currently in Select mode.

To select points, drag the left mouse button down and to the right to draw a box, the data points that fall within the box area are selected. To select points all the data points on the plot, drag the left mouse button up and to the left. A square with green outline is drawn around the data point to indicate it is selected.

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## Unselect Points

To take the mouse into unselect mode, click on the "Unselect" icon represented by

The "Unselect" icon represented by in depressed mode indicates that the mouse is currently in Unselect mode.

To unselect points, drag the left mouse button down and to the right to draw a box, the data points that fall within the box area are not selected anymore. To unselect points all the data points on the plot, drag the left mouse button up and to the left.

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## Clear All Selections

To clear all the selection on the plot, click on the "Clear all" icon represented by

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## Overlaying Plots

To overlay plots (simultaneously viewing multiple plots with similar range on the same axes system)

1. Select column to be plotted on the X-axis.
2. Select column to be plotted on the Y-axis.
3. Click "Plot" button.
4. Select column to be plotter on X-axis (represents another plot).
5. Select column to be plotter on Y-axis (represents another plot)
6. Set the "Overlay" option.
7. Click "Plot" button.

You can see the second plot overlaid on the first one if the second one is in the same range as the previous one. Only that portion of the second plot is visible that is within the already plotted range.

An example of overlaid plots is shown in Figure 6.

Figure 6

Click to see larger image.

To see the plots overlaid together (even if they don’t lie in the same range) click on the fill icon represented by . Figure 7 shows the complete datasets of all the overlaid plots in the graph above. A different marker will be used for each plot, to allow one to differentiate between the plots.

Figure 7

Click to see larger image.

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## Overlaying Histograms

To overlay histograms (simultaneously viewing multiple histograms with similar range)

1. Select column to be plotted on the X-axis.
2. Click "Histogram" button.
3. Select column to be plotter on X-axis (represents another plot)
4. Set the "Overlay" option.
5. Click "Histogram" button.

The second histogram is seen only if it lies within the same X and Y range as the first one. Click on the fill icon represented by to see both histograms irrespective of the range. A different color will be used for each histogram, to allow one to differentiate between the histograms.

Example of overlaid histogram is shown in Figure 8.

Figure 8

Click to see larger image.

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## Transformations on columns

One can create new columns by defining transformations on them. You can use expressions with arithmetic operators, trigonometric functions, and miscellaneous functions shown below to create new columns. You can use transformed columns for plotting.

### Operators Allowed

+ | Addition
- | Subtraction
* | Multiplication
/ | Division

### Functions

log(a) | Log to the base 10.
ln(a) | Natural Log of “a” with base e, where e is Euler's number (i.e. 2.718...).
pow(a,b) | "a" raised to power of "b".
sqrt(a) | Square root of "a".
exp(a) | Returns the exponential number e (i.e. 2.718...) raised to the power of “a”.
dexp(a) | 10 raised to the power of "a".
cos(a) | Trigonometric cosine of an angle. a = an angle in radians.
acos(a) | Computes the arc cosine of “a”. “a” must range from -1 to 1; the resulting angle is in radians and will range from 0 to pi.
sin(a) | Trigonometric sine of an angle. a = an angle in radians.
asin(a) | Computes the arc sine of “a”. “a” must range from -1 to 1; the resulting angle is in radians and will range from -pi/2 to pi/2.
tan(a) | Trigonometric tangent of an angle. a = an angle in radians.
atan(a) | Computes the arc tangent of “a”. The resulting angle is in radians and will range from -pi/2 to pi/2.
toradians(a) | Converts an angle measured in degrees to an equivalent angle measured in radians.
todegrees(a) | Converts an angle measured in radians to an equivalent angle measured in degrees.

##### Creating new columns

1. To define transformations on columns click on the Transformations Icon represented by to display the Transformations Dialog Box.
2. In the Transformations Dialog Box, you can see the column names, UCD and their Id (used for expression building) and the expression with which they were created.
3. For creating a new column, type in the new column name (example col2) and the expression using the functions shown above. Example: $4+log($6)

The dialog box for creating new columns is shown in Figure 9.

Figure 9

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## Creating data subsets by applying filters

You can create new data subsets by defining filters on them. You can use a condition with relational operators and logical operators with operators and functions shown above to create new data subsets. Data subsets can be used for plotting.

##### Operators Allowed

< | Less than
<= | Less than or equal to
> | Greater than
>= | Greater than or equal to
== | Equal to
!= | Not equal to
&& | And
|| | Or
! | Not

##### Creating new data subsets

1. To define new data subsets on columns click on the Filters Icon represented by to display the "Create new data subsets" dialog box.
2. In this dialog box, you can see the column names, UCD and their Id (used for expression building) and the expression with which they were created. You can also see the data subsets that you previously created with their names and conditions.
3. For creating a new data subset, type in the new subset name (example f1Glat) and the filter definition using the functions and relational operators shown in table above. You can also make use for arithmetic operators and functions listed for "Transformations on columns". Example: $1 > 0 || ($3! = 0 && $2 > 100)

The dialog box for creating data subsets is shown in Figure 10.

Figure 10

Once a data subset is created you can plot data from the subset. This can be done by choosing the data subset from the Filters combo box in the main applet window.

Note: "All" represents the complete data. Only data points satisfying the filter condition will be considered for plotting.

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## Statistical Functions on Plotted Data

Statistical functions can be applied on the plotted data.

##### Applying Statistical Functions

1. To apply statistical functions, click on "Statistics for the plot" icon represented by . The "Plot Statistics" dialog box will open.
2. You can choose the data subset from the filter combo box. Note: "All" represents the complete data.
3. Select X, Y and Z axes and click "Calculate" button. Note: Default columns are picked up from the current plot. The results are displayed on the same window.

The dialog box is divided into two tabs – basic and advanced functions. The basic functions require only one data array, while the advanced functions require two or three data arrays as parameters.

The following statistical functions are currently supported.

Sr. No. | Function name | Input Columns
1 | Number of observations | X
2 | Range | X
3 | Minimum | X
4 | Maximum | X
5 | Mean | X
6 | Variance | X
7 | Standard deviation | X
8 | Skew | X
9 | Kurtosis | X
10 | Linear correlation | X, Y
11 | Significance (t) for Linear correlation | X, Y
12 | Probability for Linear correlation | X, Y
13 | Rank correlation | X, Y
14 | Partial correlation | X, Y, Z

VOPlot, by default, takes only the only the plotted data range into consideration while evaluating the statistical functions. This will evaluate the statistical functions for the currently plotted columns, along with the applied filter, if any, in the current axes ranges. However you can force it to consider complete dataset by selecting the "Entire Data" checkbox. Similarly, you can force it to consider the selected data range by selecting the "Only Selected Point Data" checkbox

Sample dialog box with basic tab selected showing plot statistics is shown in Figure 11.

Figure 11

Sample "Advanced Functions" tab is shown in Figure 12.

Figure 12

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## Display VOTable

Used to display Plot data in VOTable format. It also displays the filters and data sets that are user-defined.

##### Displaying the VOTable:

To view the VOTable, click on "Display VOTable" icon represented by . The "Display VOTable" dialog box will open.

##### Selected Data:

The ‘Display VOTable’ dialog box by default displays the metadata and the data corresponding to the selected data points on the graph.

The entire VOTable data can be viewed by removing the selection from the checkbox “Display Data Of Selected Points”.

Note: The data displayed is truncated to the first hundred points.

Sample dialog box displaying VOTable containing data of selected points is shown in Figure 13 below.

Figure 13

Click to see larger image.

##### Entire Data:

The ‘Display VOTable’ dialog box by default displays the metadata and the data of the data points on the graph if points are not selected.

The selected points VOTable data can be viewed by selecting the checkbox “Display Data Of Selected Points”.

Note: The data displayed is truncated to the first hundred points.

##### Save VOTable:

This feature is available only in the stand-alone version of VOPlot. To save a VOTable, click on “Save As File” button on the dialog box. The save as dialog box will appear. Select a file and click on OK. The file is saved as an XML file, VOTable format. The various user-defined filters and data columns pertaining to all points (more than 100) are saved in the file.

Note: This feature is not available in the web-based version of VOPlot. There is no “Save As File” button on the dialog box as shown in Figure 14 below.

Figure 14

Click to see larger image.

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## Display Data

The VOTable data used to plot the graph can be displayed in tabular format. It also displays the filters and data sets that are user-defined.

##### Displaying the VOTable Data in Tabular Format :

To view the VOTable data, click on "Display Data" icon represented by . The "Display Data" dialog box will open.

The data is displayed in two different tables. The first table displays the field metadata of the VOTable, while the second table displays the actual data.

If points are not selected on the plot, the entire plot data is displayed.

Sample dialog box displaying the selected data and metadata of the VOTable is shown in Figure 15.

Figure 15

Click to see larger image.

##### Selected Data:

To view the data of the points selected on the plot. This dialog box shows data of selected points by default (if user has selected points); otherwise data of all points is displayed.

The selected points data can also be viewed by selecting the radio buttons “Show Selected”.

##### Entire Data:

To view the data of all the points in VOPlot. If points are not selected on the plot, by default the entire data is displayed.

The entire data can also be viewed by selecting the radio buttons “Show all”.

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## Saving plot as an image

In the standalone version of VOPlot, for saving the plot as an image, plot the desired image and click on save icon represented by . The save as dialog box will appear. Select a file and click on OK to save the image as an EPS file onto your machine. Currently, the image can only be saved as an EPS file.

Note: This feature is not available in the web-based version of VOPlot.

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## Printing plot

For printing the plot, plot the desired image and click on print icon represented by .

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##### Feedback Address

For feedback on VOPlot contact voindia@vo.iucaa.ernet.in.

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