Allow tuples to define the linetype in element_line

[concimuscb]

Hello,

It would be interesting to add the possibility of passing tuples to the linetype argument to define custom linetypes.

I quite like the plotnine implementation: (4, (1, 4, 10, 3))

The first element is always an integer which defines the offset to start drawing the line in relation to the axis of the chart.
The second element is a tuple of n elements. The first element is ink, second is space, third is ink, fourth is space, and so on so forth.

In the example a line is drawn with 4 (px?) off set and then 1 px ink, 4 px space, 10 px ink, 3 px space and then the sequence is repeated 1,4,10,3....

Not sure how challenging would it be to implement this.