rqt_plot: don't add more padding for each additional curve
rqt_plot: don't have required minimum padding

With a fixed absolute minimum padding, rqt_plot is useless for plotting
arbitrarily small values. (For example, trying to plot a magnetic field
measured in teslas, usually having a magnitude of about 1e-4.)
Merge pull request #223 from forrestv/zero_abs_padding

rqt_plot: decreased required minimum padding to 0