3.2 A plot window

The anatomy of a plot window is shown below:


It has three main regions, the menubar & toolbar, the control panel and the display area.

The display area

The display area shows a view of all the spectra that are currently associated with this plot. The colours, plotting style etc. of these spectra are determined by their global properties as set in the main browser window.

The plot title and axis labels are determined using the properties of the current spectrum, if possible. For instance if the current spectrum is an NDF then these values are determined from the NDF’s title, units and axis components, or for more recent NDFs from the properties of its WCS component. Similarly for FITS files these values are determined using the world coordinates FITS keywords. To change these look in the Configure Plot Options window, which is activated by pressing the PIC button.

The size of the area used to display a spectrum is initially determined from an automatic match of its data and coordinate extents to the visible surface of the display area, you can change the size of the display surface using the zoom controls described below and/or you can change the actual data limits using the Configure Plot Options window. When you add or remove a spectrum the apparent size of the display region stays the same and all the spectra now displayed are scaled to fit within it. To return to a state where the visible surface matches the display area you need to press the PIC and PIC buttons.

The Configure Plot Options window also offers the ability change the colour of the display area background, to draw the lines using anti-aliasing (which reduces the jagged effects, but at the cost of drawing speed), to modify the reserved area around the plot and do things like position the numeric labels, set the axis gaps etc. 

The following interactions are provided in the display area.

The control panel

The control panel area provides controls for interacting with the plot. It also shows details of what spectra are displayed in the plot and a continuous readout of the X coordinate and corresponding data value.

The Displaying: drop down list shows the names and current rendering properties of any spectra that are displayed. If you choose a spectrum from this list it becomes the current spectrum and is used as the basis for the plot’s coordinates (so the readouts now show the values of this spectrum, also the plot will be re-drawn if the coordinate system of the newly selected spectrum is different). The selected spectrum also becomes selected in the browser window (so you see its details more clearly and apply modifications). This spectrum is also the one used by any tools (such as background and line fitting) that work with only one spectrum. If you remove this spectrum using the Edit->Remove current spectrum option then the next spectrum in the list becomes the current one.

The Y limits (%): drop down list provides a series of quick cuts for setting the limits of the Y axis. Normally this is the full range, but using a percentage cut can usefully clip the range to reject extreme outlier data values. The data values used are restricted to the limits of then X coordinates (all when automatic ranging is used, the default ).

The Wavelength: entry control has two functions. The first is to show the current wavelength. The second is to allow you to centre the display area on a specific coordinate. Just type in the value you want to see here and press <Return>. Note that the label of this area modifies to match the label of the X axis, so could say something different to Wavelength:.

The :V-hair checkbox controls whether the ‘vertical hair’ is shown. The vertical hair is just a vertical line that follows the mouse pointer around the display area. It can be quite slow, so isn’t shown by default.

The X scale: and Y scale: controls change the zoom of the display area. The plus and minus controls increase and decrease the zoom by one, or you can choose a zoom from the drop-down lists, or you can type in a decimal value (which should be greater than equal to one) and press <Return>.

The :log controls determine if either of the axes are drawing using log spacing (this is not possible if either axis spans the value zero). Finer control of the log spacing and labelling can obtained using the plot configuration window.

The Track free checkbox determines what values are shown in the continuous coordinate readouts. When unchecked (the default) the values shown are the nearest ones of the current spectrum to the pointer position. When checked the readouts show the coordinates under the pointer.

Toolbar and menus

The toolbar region has a series of buttons that act as short-cuts to most of the functions found in the main menus. Some of these create new windows with complex actions that should be consulted for further help.

The Options menu contains several control items whose values are preserved between invocations of SPLAT-VO.

The Graphics menu

The Graphics menu provides methods for drawing text and a range of figures on the plot. This allows you to add annotations, but also provides the basis for the interactive graphics used by the various toolboxes.

To draw a figure on a plot you need to select an option from the Drawing mode sub-menu and then, depending on the figure type, either drag out a region or select fiducial points. When creating a figure using points these are completed using a double click on the last point.

In the Drawing mode menu the currently available figure types are:

There are also two special modes:

When the mode is select (this is usually the default state), clicking on a figure selects it. You can then drag it around to move it, or drag one of the grips (the little squares) to change the shape. To select more than one figure you hold down the <Shift> while clicking.

The edit mode can be used to add new points to a polyline, polygon or interpolated curve. Just select this, click on the figure you want to edit and then start adding new points. You can also edit text strings, just choose the edit mode and click on the text, this produces a dialogue with the text for editing.

To change the properties of a figure (line width, fill/outline colour, font etc.) just select the figure and then choose the option you want to change from the various menus. You can define these before creating a figure too.

Interpolated curves are like polylines in that they are defined by a series of points, but they can only have increasing or decreasing X coordinates (so cannot double back on themselves). They are intended for drawing figures that are related to spectra, specifically spectral backgrounds and lines. In fact if you draw an interpolated curve you can convert it into a spectrum for removing from a real spectrum using the Generate spectra from interpolated lines toolbox. The types of interpolation scheme available are:

Once you have created your figures you may want to save them for restoration at a later time, this can be done using the Graphics->Save/restore figures dialogue. Figures are saved using the physical coordinates of the spectrum, so should be re-drawn at the correct wavelengths etc. on new plots.

Accelerator keys