Correcting bad string comparsion in lin-log plot aspect verification

Correction to little bug in the aspect verification

If trying to plot a lin/log, it wrongly raises the following warning

    matplotlib/lib/matplotlib/axes/_base.py:1148: UserWarning: aspect is not supported for Axes with xscale=linear, 
    yscale=log 'yscale=%s' % (xscale, yscale))