Limit xrange when using xlen

Hi,

Here's a very simple fix to avoid the fact that, when using xlen, the plot is initially mostly empty. It seems more logical (at least for what I'm doing) to have its xrange be initially small and grow up to xlen.

What do you think about this? Maybe the best would be to have an option to choose between the two possible behaviors?

Cheers,

-- 
Antoine