Problem with a QwtPlot with big axes values

[Uwe]

An infinite step size doesn't happen somehow - you have assigned it explicitly. So this is your code:

if ( !qIsFinite ( stepSize ) )
    stepSize = -1;
 
plot->setAxisScale( ..., stepSize ):

if ( !qIsFinite ( stepSize ) )
    stepSize = -1;

plot->setAxisScale( ..., stepSize ):

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It is not the job of Qwt to deal with any type of nonsense the application code does. When passing a double it has to be valid otherwise the result is undefined.

Uwe