I guess this would be a way to do it in case you really
want to change data of an existing plot:
execute this code first to get a figure:
# generate a simple figure
f, ax = plt.subplots()
ax.plot([1,2,3,4,5], [5,6,7,8,9])
ax.set_xlim([0, 10])
ax.set_ylim([0, 10])
and then use this to switch the data of an existing line
# get data from first line of the plot
newx = ax.lines[0].get_ydata()
newy = ax.lines[0].get_xdata()
# set new x- and y- data for the line
ax.lines[0].set_xdata(newx)
ax.lines[0].set_ydata(newy)