On 4/2/13 9:03 PM, hibou21@gmail.com wrote:
> I am having a strange IDL problem involving logarithmic axes. I am trying to add a logarithmic axis on the right hand side of a plot using the AXIS command. The function that relates the values on the left-hand-side to those on the right-hand-side is:
>
> (90.0/3.85)*(10.0d^(!Y.CRANGE/2.50d) - (90.0/3.85)
> (The value 90.0/3.85 ~= 23.4)
>
> When I try to plot (90.0/3.85)*(10.0d^(!Y.CRANGE/2.50d), without subtracting the final number, IDL does what it's supposed to do.
>
> However, when I try to plot my full expression:
> (90.0/3.85)*(10.0d^(!Y.CRANGE/2.50d) - (90.0/3.85)
> IDL gives the error:
> % AXIS: Warning: Infinite plot range.
>
> I played around with it a bit and it turns out you can subtract up to about 14.8 alright, but once you try subtracting more than that, it stops working and gives the error.
>
> Here is a sample plot that you can use to reproduce the error:
> IDL> plot, [1,2,3], [1,2,3], yr=[-0.5,4.5],ysty=9
> IDL> AXIS, YAXIS = 1, /ylog, YRANGE = (90.0/3.85)*(10.0d^(!Y.CRANGE/2.50d))-15, /ysty, /save
>
> I've tried resetting !Y.Type to both 0 and 1, as suggested online, but to no effect. I've also tried inputting the YRANGE values manually [-8.6269713,1451.5886], but it doesn't make a difference. I'm really stumped by this...
>
> My IDL information: IDL Version 7.0, Mac OS X (darwin i386 m32). (c) 2007, ITT Visual Information Solutions
>
There are a few problems here, and they mainly boil down to your use of
/YLOG.
/YLOG can be used to have a uniform logarithmic axis. But that's clearly
not what you have, because of that offset. And because /YLOG expects the
YRANGE to be in terms of the actual number (not the logarithm), when it
sees a negative value, it dies since you're effectively telling it that
you want the y axis to extend down to -infinity.
What I generally do when I have an arbitrary transformation between two
different axis ranges is to stick the tick marks on manually. I can't
tell based on what you've said what the actual minimum value that you
want plotted is, but say it is 0.1 and you want tick marks at [1, 10,
100, 1000, 10000]:
ytick_values = [1, 10, 100, 1000, 10000]
c_offset = 90.0/3.85
; inverse transformation of what you gave, so this will
; take RH axis values and turn them into LH axis values:
ytick_transformed = 2.5*alog10((ytick_values + c_offset)/c_offset)
; make an axis that matches the original LH y axis by using the same
; y range and forcing the y style to be correct, but puts RH axis
; labels at the appropriate LH locations
axis, yaxis=1, yrange=!y.crange, ystyle=1, $
yticks=n_elements(ytick_values)+1, ytickv=ytick_transformed, $
ytickname=string(ytick_values,format='(F0.0)')
-Jeremy.
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