bar_plot takes one mandatory argument and one optional argument:
M, a matrix, where each row consists of a label followed by one or
more values. If the labels are followed by more than one value,
then the first row needs to be identifiers.
color=col, where col is a color object
or a list of color objects (see Section 18.1.3). By default,
col=black.
bar_plot(M ⟨,color=col ⟩)
draws a bar graph with a bar for each label, whose height is given by the
corresponding value. If the matrix has more than two columns, then there
will be a bar plot for each column of values. If col is a color object, then
the bars are filled with the specified color. If col is a list of colors,
then the fill color cycles through the list for bars from left to right (each group
of bars is colored by using the same colors).