Graphing tab: refrain from plotting every timestamp

[shape-warrior-t]

TelemetryData.get_parameter_values provides a step argument, as not every timestamp is necessary to plot when graphing over a long time range. Currently, the method is always called with step=1 -- for efficiency reasons, this will likely need to change somehow (either automatically calculating step in some manner, likely based on the number of telemetry frames and also potentially the number of parameters being plotted, or moving to an entirely different way of solving the problem).