DPlot

Hi Dave;
I'm utilizing DPlot on a regular basis. I am working on a hydrology based flood frequency analysis (FFA), where the probability of exceedance on the abscissa is plotted against Flow Discharge (Q) on the ordinate axis. I use Scaling Options G & H for the plots. Not an issue.

However, today's dumb question concerns generating a 2nd x-axis (along the top of the plot) that would show the probability values converted to their equivalent recurrence intervals (or return period). Recurrence intervals are the inverse to the probability of exceedance values (eg. if the probability value = 0.2, the equivalent return period is 5.0 years in this case).

As you probably know, return period is a common parameter that is used for expressing the magnitude of a "design discharge" for engineering related applications. But under the "multiple axes" option, it appears that the function allows only manipulation of the coefficients & not the variable (X1) which I would like to show as the inverse on the second x-axis (or on the single default x-axis)

Please confirm if I am interpreting the "multiple axes" option properly and whether or not a stategy exists for resolving this situation.

In advance, I thank you for your response to my question.

Regards,

RWA.

Bob,

Please confirm if I am interpreting the "multiple axes" option properly...

Yes.

and whether or not a stategy exists for resolving this situation.

Unfortunately no, but this should be easy enough to fix in a future release. There are really two problems - the multiple axis option is ignored for probability scales (and really it ought to be disabled to avoid confusion). The second X axis will always show 100-x regardless of what values you use here. I'll most likely throw out that "feature", which can be restored easily enough with the multiple axis setting.

The second problem is one you've already noticed - the relationship between the values on the axes can only be linear. What's needed is a more generic equation - in your case x' = 100/x. This shouldn't be too much of a problem; I'll try to have it in the next release (Y axis too, of course).