Function to generate a scale for heatplots
colour.bar(col.data, min, max = -min, cex.ticks = 1.2, nticks = 11,
ticks = seq(min, max, len = nticks), title = "", ylab = "",
cex.lab = 1)vector of colours
minimum colour
maximum colour
size of ticks on colour bar
number of ticks on colour bar
vector of positions of ticks on colour bar
optional title for colour bar
optional y-axis label for colour bar
size of labels on colour bar