Coyote's Guide to IDL Programming

Forcing Exponential Axis Annotation

QUESTION: It seems that IDL has some kind of internal algorithm that sometimes selects exponential axis tick labeling and sometime selects number axis tick labeling. Can I force it to use exponential tick labeling all of the time?

ANSWER: You can create tick labels any way you like by using a special tick labeling function. The function is called by specifying the name of the function using the [XYZ]TickFormat keyword to a graphics command.

Here is a tick label function for forcing exponential axis labeling written by Stein Vidar Hagfors Haugan. Notice that the function is defined with three positional parameters. These are required for a tick formatting function.

   FUNCTION Exponent, axis, index, number

     ; A special case.
     IF number EQ 0 THEN RETURN, '0' 

     ; Assuming multiples of 10 with format.
     ex = String(number, Format='(e8.0)') 
     pt = StrPos(ex, '.')

     first = StrMid(ex, 0, pt)
     sign = StrMid(ex, pt+2, 1)
     thisExponent = StrMid(ex, pt+3)

     ; Shave off leading zero in exponent
     WHILE StrMid(thisExponent, 0, 1) EQ '0' DO thisExponent = StrMid(thisExponent, 1)

     ; Fix for sign and missing zero problem.
     IF (Long(thisExponent) EQ 0) THEN BEGIN
        sign = ''
        thisExponent = '0'

     ; Make the exponent a superscript.
     IF sign EQ '-' THEN BEGIN
        RETURN, first + 'x10!U' + sign + thisExponent + '!N' 
     ENDIF ELSE BEGIN             
        RETURN, first + 'x10!U' + thisExponent + '!N'

To see how it works, first try this command:

  IDL> Plot, [1, 1e3]

And then try it with the tick formatting function, like this:

  IDL> Plot, [1, 1e3], YTickFormat='exponent'

Here is an example that shows how the Y axis labels should appear after typing the two commands above.

Using EXPONENT function
Example plot with and without EXPONENT function tick formatting.

This function can even be used with logarithmic axes, like this:

  IDL> Plot, [1,1e3], YTickFormat='exponent', YLog=1
Configuring Log Plots
The EXPONENT function can also be used with log plots.

Paul van Delst has written a similar tick formatting function, that he describes in this IDL Newsgroup article.

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