You may apply a tiny hack to domoticz\www\js\domoticz.js on your webserver.

In function ShowCounterLog() around line 5902 change line
type: 'column',
to
type: 'spline',
and in line 5907 change
$.getJSON("json.htm?type=graph&sensor=counter&idx=" + id + "&range=day",
to
$.getJSON("json.htm ...