bar_plot takes one 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.
bar_plot(M) 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 graph for
each column of values.