This function should not be used directly,
only inside .plot_boxgraph_RNAratio_TCGA()function.
Side effects: (1)the box plots on screen and as pdf files to show differential RNA ratios across tumors
.RNAratio_boxplot(df, dashline, ylimit, filename)output dataset generated from .RNAratio_selection() function
the position of horizontal reference line
the lower and upper limit of the axis scale
the name for the output file
Other helper function for differential expression plotting:
.RNAseq_boxplot(),
.RNAseq_grouped_boxplot(),
.violinplot()