IDL> x = findgen(100)/20.
will generate an array of floating point real from 0 to 99. y will then contain
Even if you don't have the x values, they will be replaced by a pixel scale:
You can easily overplot:
There is a lot of different art of plotting, you can fix the style, the size and/or the color of the drawing lines, set the x- and y-axis range, plot in log-scale, and so on. Just have a look into the manual pages at plot.
For example, try the following plotting commands:
IDL> x = findgen(200)*10
IDL courses C. Morisset © 2004 IA/UNAM V 2.2