How to best visualise two sets of data over several years?
Statistics of this nature are commonly normalised with respect to the population sizes by presenting them as per X. In this case per 1,000 or 100,000 people.
Absolute number of crimes makes no sense and lacks context. In terms of display, as both KPI ranges are similar (3 vs 19) a common X and Y axis is possible with two different lines.
If you want to communicate trends relative to a baseline year (e.g. 2014) you could divide all values relative to this year and express it as a % change.  I.e. crime went up 10% but police numbers went down by 5%.