Beginner's IDL - Line Plots III

• Adding Text to Line Plots - Brief Recap
• Positioning Graphics in the Display Window
• Handling Missing or Bad Data in Line Plots

Adding Text to Line Plots - Brief Recap

In Line Plots II, we have learned how to specify the type of fonts and the position of a string placement in the xyouts procedure. The same set of font-manipulation commands is also applicable to any character strings. You can use the same set of commands to display different fonts and different positionings in the plot titles. For example, below we will plot out the spectrum from the previous exercise and format the units for the x-axis title.

IDL> plot, velocity, smoothed_flux, xrange=[-2000,2000],xstyle=1, $
IDL>yrange=[-5.e-16,10.e-16], $
IDL>ystyle=1, title='GHRS Spectrum, velocity vs smoothed flux', $
IDL>xtitle='Velocity (km s!E-1!N)', ytitle='flux'

Positioning Graphics in the Display Window

IDL has a number of ways to position line plots and other IDL graphical displays in the display window. The plot position may be set by !P.Position system variable or by the Position keyword to the Plot command. Here we will only explore the method to create multiple plots in a single display window or graphic device.

In IDL, the !P.Multi system variable can be used to create multiple plots in a display window. !P.Multi is a five element vector defined as follows:

!P.Multi(0)	Contains the number of plots remaining on the page. Start with this as 0 to clear the page.
!P.Multi(1)	The number of plot columns on the page.
!P.Multi(2)	The number of plot rows on the page.
!P.Multi(3)	The number of plots stacked in the Z direction.
!P.Multi(4)	If 0, plots are displayed from left to right and top to bottom, i.e., the plots are displayed in rows. If 1, plots are displayed from top to bottom and from left to right, (i.e., the plots are displayed in columns).

To display four plots on a page in two columns and two rows, and the plots should appear in columns, the !P.Multi array should look like:

IDL> !P.Multi = [0, 2, 2, 0, 1]

Now as you display each of the four plots, each plot fits into a quarter of the display window:

IDL> plot, num, line, linestyle=0
IDL> plot, num, line, linestyle=1
IDL> plot, num, line, linestyle=2
IDL> plot, num, line, linestyle=3

To reset back to display a single plot on a page, the !P.Multi should be set to 0.

IDL> !P.Multi = 0

Exercise: See if you can plot the original and smoothed spectra side by side.

Handling Missing or Bad Data in Line Plots

In IDL, you can use the IEEE standard value NaN (not a number) to denote bad or defective values in your data sets. The NaN values are stored in the system variable !Values. To assign the value to a variable:

IDL> badvalue = !Values.F_NAN

Example: Here we will create a dataset with seven random bad values:

IDL> badData = line
IDL> badData(Fix(RandomU(seed, 7)*100)) = !Values.F_NAN

You can plot this bad data by typing:

IDL> plot, badData

When you plot a dataset containing NaN, the bad values are simply dropped out.


Another way to handle defective data is to screen it out with either the Max_Value or Min_Value keywords to the Plot command. The keywords do not perform the screening, and so you will have to do the screening yourself. If you know the exact value for the bad data, you can select them using the where function. Below demonstrates how to select all values between -0.15 and 0.15 as bad values. After the selection process, you could set the values to some value greater than the real data values and screen them with the Max_Value keyword.

IDL> badData = line
IDL> badValues = where(badData gt -0.15 and badData lt 0.15)
IDL> badData(badValues) = 100.0
IDL> Plot, badData, Max_Value = 1.0