Wednesday, 11 September 2013

Disappearing log plots and lines

I was doing some plotting using iPython --pylab which uses the handy matplotlib to draw things. The stuff I was plotting was a bit all over the place so I thought I'd switch to log mode using pyplot.yscale() (though you can also use axes().set_yscale()):
Suddenly a few of my lines just went missing - What was the problem here ?

Basically it comes down to the fact that some lines/points were at zero which is problem for a logarithmic representation as the log of zero is -infinity (-∞). Of course iPython has a work around which is to use their symlog axes scale, which combines log and linear scales using a threshold linthreshy at which they change:
You can also have a problem with negative values and log plots as the log of a negative number cannot be represented in normal (real) numbers (use need complex numbers for that). But you can get around that in iPython using the symlog as well - as explained here.
Note: All these commands work for the x axes as well - just change the 'y' for an 'x'.