Update URL for links to ggplot

[andreas-h]

In many places in the documentation, when referring to the original ggplot library, the URL http://had.co.nz/ggplot/ is given for a link. However, this URL seems to be obsolete, I get a 404 error. The new website seems to be http://ggplot2.org/. Maybe it would be a good idea to change the mpl docs accordingly.

Is there some macro defined somewhere, or do we need to grep on the whole source tree?

[tacaswell]

I think this is already done in the source, but the main pages have not been re-built see #6678

[tacaswell]

Can you check the docs at http://matplotlib.org/devdocs and http://matplotlib.org/2.0.0b3 ?

[QuLogic]

Based on #6981 getting filed, I'd say this change has not been made everywhere yet.

[tacaswell]

Because the main-page docs have not been re-built yet. This was closed by #6861

[QuLogic]

No, #6861 only covers the example files; the prose still contains these links.

[jenshnielsen]

Closed by #6986