Examining the graph, we can see that there can't be a proportional relationship between time and the height of the snow.
Proportional relationships, graphically behave as a linear function with no linear coefficient like this:
What we see in the graph, is a period of stability over time. So we can not tell this a proportional relationship.
If it was, as time goes by so the height of the snow would grow. For example:
t h
1 2
2 4
3 6
4 8