TAChart: Logarithmic diagrams
lazarus / 55-TAChart

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wp · 2011-08-15 16:30

When plotting data on logarithmic axes the logs have to be calculated
explicitly before adding the data to the series. This is quite
unusual: Excel, gnuplot etc. accept direct data and calculate the logs
themselves. This would make life easier when an applications allows
the user to switch between linear and log scales.

I fear that some internal routines might get screwed up, but is it
possible to have that behavior in TAChart as well? I mean that the
ChartTransformation should be responsible for calculating the logs of
the data, not only of the axis labels.

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Ask · 2011-08-15 17:34

QuoteWhen plotting data on logarithmic axes the logs have to be
calculated explicitly before adding the data to the series.
Why do you think so? Please look at axistransf demo -- does it not
work for you?

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wp · 2011-08-15 21:08

Enclosed is a simple project plotting the function y = exp(-x). On a
semilog scale the exponential curve must become a straight line -
that's simple math. In this program, however, this is true only if the
two lines marked by the comment "Needed" are active.

What am I doing wrong?

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Ask · 2011-08-15 23:01

You did not set AxisIndexY property.
See http://wiki.lazarus.freepascal.org/TAChart_documentation#Axis_tran
sformations

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wp · 2011-08-16 18:46

Ah! Guess I should read the docs once more...