Equal scaling of axes

[13bzhang]

This pull request is a solution to fix Issue #218 "Equal scaling of axes not working". I fixed it so the width and height of Plotly plots have the ratios specified by ggplot2 user if they decide to specify coordinate ratios.

[mkcor]

Aww a .Rhistory file should never end up in a PR... Please use

$ git add <file>
$ git commit -m "commit message"

(suspecting you used git commit -am ...?) but a .Rhistory file should never be tracked by your VCS in the first place. I'm not sure it's enough to remove it with an additional commit, because I wouldn't want it anywhere in our commit history.

@13bzhang Can you please submit a new PR and I'll close this one? Thank you so much!

[mkcor]

Please add spacing around * operators and... I think the dotted notation is not our friend for simple variable names (the R culture is changing too: http://kbroman.org/hipsteR/).