What black lines on graph means?
They are at times when the values are zero (or possibly missing) so the colour is not filled in. Zoom in and you will see the gaps.
Zoom in closer, on one of the wide gaps, and have the two graphs with the same time scale please.
Did that help at all?
1 Be sure in your database you are not missing data point.
2 On the graph setting ==> visualization ==> Stacking & Null value ==> Null value = connected