[vislib] Point series charts hould have bottom line

[w33ble]

Currently, if the Y-Axis drops below the 0 value (any time we graph negative values), we don't draw a line at the bottom of the chart, just a line at the 0 value.

Additionally, if we set custom extents so that there is no 0 value, we get no line at all.

I think we should always draw a line on the bottom of point series charts, and additionally draw a line at the 0 value if it's not at the bottom of the chart.

[ppisljar]

fixed by #9044