Python heatmap: Change colour map and make asymmetrical

This will solve your first two problems -

fig = plt.figure()
fig, ax = plt.subplots(1,1, figsize=(12,12))
heatplot = ax.imshow(data_matrix,cmap = 'Greens')

cbar = fig.colorbar(heatplot, ticks=[data_raw.overlap.min(), data_raw.overlap.max()])
tick_spacing = 1
ax.set_title("Overlap")