colorbar: format labels with mathtext

Hi Everyone,

I would like to format the labels of a colorbar with mathtext. I have
tried a lot with all the cax.formatter.___ options but unfortunately I
was not able to get it working.

I am plotting some Data with
   col = ax.pcolorfast( ... )
after this I create a colorbar with
   cax = fig.colorbar(col)
Then I would like to change the formatter and the powerlimits with
   cax.formatter.format = '$%1.2f$'
   cax.formatter.set_powerlimits( (-4,4) )
but this does not work for me.

Could you pleas help me to set the powerlimits and to format the labels
of a colorbar with mathtext.

Regards,

Stefan