#1413 Plot: Plotting in time shows no data when 1 only timestep selected

[Scott]

When making a plot with time as the independent variable, no data shows up if only one timestep is selected. We should either draw an impulse for that timestep, or force the user to select more than one timestep at a time.

To reproduce:
1) Load any dataset, open the plot tool
2) Under the time tab, set the Min time slider to be equal to that of the Max slider
3) Add a variable and plot the data. No lines will be drawn.