Plotly.js: Specify annotations[0].align/annotations[0].valign on [0, 1] scale

[cpsievert]

Currently these attributes may be specified on a discretized scale with 3 categories.

Mainly requesting so we can achieve parity with ggplot2 when converting labels/annotations:

qplot(1:10) + 
ggtitle("Hey there") + 
  theme(plot.title = element_text(size = 40, vjust = 1, hjust = 0.75, debug = T))

[alexcjohnson]

If we did this, ideally we would standardize the implementation so it applies to everything that uses these discrete categories:

• xanchor/yanchor for annotations, legends, images, colorbars, rangeselectors, sliders, updatemenus
• scatter.textposition