In the documentation of matplotlib.lines.set_linestyle() and elsewhere in the matplotlib docs it is written that the linestyle can be a dash tuple of the form (offset, onoffseq), where onoffseq is an even length tuple.

Providing the onoffseq works fine, but the offset seems to be ignored, e.g., the following code produces two identical plots:

x = linspace(0,10)
plot(x, ls=( 0, (10,10)))
plot(x, ls=(10, (10,10)))