Figure 1. Time-series graph from the 10th or possibly 11th century [Unknown, 1010] (a) As you can see, the image has no labels on the zr- or y-axis. What do you think the :r-axis and y-axis represent? (b) This graph is novel because a coordinate system was used to plot the various changes in orbits for the seven planets over time. Why do you think that a coordinate system was needed for this graph?