Test label width adjustments

[jnothman]

We currently try to allot an appopriate amount of space for tick labels on the matrix plot. We should have formal assertions that:

• for varying lengths of label, varying totals_plot_size and varying number of intersections
• for varying tick font/size
• there is a minimal margin between the tick label and the totals subplot
• the margin is at most the width of one matrix subplot column