use grave accents ` to allow any sample name for plot_scatter

[warrenmcg]

Hi,

In plot_scatter, aes_string from ggplot2 is used to take the log of the normalized counts for the scatter plot using the sample name as an expression. However, sample names that would cause a syntax error (e.g. using hyphens, which are interpreted as minus signs) cause an error unless the grave accent ' ` ' is used (this is the source of the error in #89).

I'm not sure if there is another case where this proposed code would break when the original wouldn't (maybe check to make sure the sample names exist in the sample_to_covariates table?), but here is the proposed solution.

Best,
Warren

[warrenmcg]

My new pull request incorporates this code, so I'm closing this pull request in favor of the new one.