fix plotting of directivity

[nicolas-f]

Fix plot of directivity graph.

Do I create a WPS block to generate cardioid ? Is someone would found this useful ?

$$ r(\phi,\theta) = {-40(1-(0.5+0.5cos(\phi)cos(\theta)))} $$

$$ r(\phi,\theta) = {-40(1-(0.5+0.5cos(\phi)cos(\theta))⁵)} $$

[pierromond]

Do I create a WPS block to generate cardioid ? Is someone would found this useful ?

Yes ! :) ahha it is a very nice idea !