A function to plot a histogram with data split into different categories
histogram(header, data, sheader, sdata, category, column)
m x 1 vector of headers for numeric data
n x m matrix of numeric data, with matrix of column vectors
p x 1 matrix of corresponding headers for string/text data
n x p matrix of string/text data with data as column vectors
label of field on which to split the data into different groups and colours for plotting
string label for column or index