Steps to reproduce:

1. update on server from 3.4 to 4.0.0 on debian 9
2. on client open firefox
3. open firebug
4. create brakpoint on string 1051
   if ((e.pageY < offset.top) || e.pageY > offset.top + data.height) {
5. one click to  graph
6. look to firebug debug for variables data.height and offset.top + data.height
7. data.height  is string, but offset.top is integer or float. Sum of them is sting with unexpected results when comparing

I reproduce this bug on ubuntu firefox 62 and one of windows 10 google chrome