[bug] Iperf bar charts time range on x-axis is inconsistent with other charts

[Aryamanz29]

Reported by @pandafy in https://github.com/openwisp/openwisp-monitoring/pull/394#issuecomment-1202118479

• If we look below stackedbar+lines charts, time interval starts from August 1 12:00 hrs with 12 hrs time difference between each points on the x-axis.

• While other iperf charts shows time interval starts from August 1 18:00 hrs with 6 hrs time difference.

Possible solution

• I found that options.mode = 'lines+markers' for the stackedbar+lines charts which streches charts x-axis to some backward time we can use options.mode = 'lines' for iperf charts to avoid this.

Note : Iperf charts uses connect_points chart property which connects only non-null values by filtering out it's existing x-axis & y-axis, therefore their start time point on x-axis is different from normal monitoring charts but all iperf charts have consistent x-axis intervals.