Imagine that this were a time series graph of your body temperature over the last week. With the Y-axis set at (min_temp, max_temp) or similar, the graph would clearly show a mean of about 98.6F and consistent trends based on the hour of the day. If you had a fever (>100F), it would be readily apparent in the graph.

A zero baseline for this chat is meaningless and would render the plot useless, unless the purpose of the graph was to show what happens to your body temperature as you turn into a White Walker.

Similarly, the axis chosen for the mortality chart is effective since it shows the typical range of values (and thus does convey the scale effectively), the consistent seasonal trends, and the anomaly (which is very significant in terms of standard deviations above the mean). Setting a zero y-axis would squeeze the graph into 1/3 of the vertical space, leaving 2/3 as useless white space for no benefit at all.