Support for specifying legend/axis font size via theme()

[Faye-yufan]

Currently Animint theme() does not support arguments like text, axis.text nor legend.text to change axis/legend text size like below:

+ theme(...,
        axis.text = element_text(size=X),
        legend.text = element_text(size=X),
        ...)

I think this feature would require add new functions in extract theme settings, add_plot func, d3 axis text setting and add_legend func in animint.js. And the axis and lengend font size maybe default to 11px and 12px seperately if user not defined.

It may require add conditions to extract axis.text params or another variable name for assigning axis text size, maybe like axis.font. For legend text size, I think simply add .style("font-size","XXpt") would work.

[Faye-yufan]

In PR#66, user configurable axis text size has enabled via defining text, axis.text, axis.text.x and axis.text.y, the syntax is the same as ggplot2. And rel() also works for the latter three, since these are the children elements of text.

Legend text size now can be defined via legend.title and legend.text. However, rel() does not work in defining this situation, since originally animint set both title and text default size to 16px and it is different from what ggplot did, which is legend.title default to 11pts and legend.text, as it inherits from title size, default to rel(0.8)(8.8px).

Besides, the relative text size in ggplot is to make sure text size does not change when user changes the dpi, maybe it is not that important in terms of output HTML.

[tdhock]

it seems there is still an issue when the global text option is specified as in the code below

animint(
  ggplot()+
    geom_point(aes(
      Foo,Foo,color=Foo),
      data=data.frame(Foo=1:5))+
    theme(text=element_text(size=30)))

I get the result below

we see that all the text gets bigger, except for the axes titles.
the expected behavior is that the axes titles get bigger in this case.
we should fix this.