SetYScaleType — Select linear or logarithmic scale
$plot->SetYScaleType($st)
SetYScaleType sets the scale type along the Y axis
to be either linear (the default) or logarithmic.
No Y value may be less than or equal to 0 with logarithmic Y scale.
Support for logarithmic scales in PHPlot is limited. One problem is that tick interval is fixed for the entire range of data, which is inappropriate for logarithmic scales where the data spans more than one magnitude.
The default X and Y scale types are linear.