plots: add an option for a 2-D plot of one dependent variable against another

[GraemeWatt]

Some heavy-ion publications have plots of one measurement against another measurement, where the measurements both have errors and therefore need to be encoded as dependent variables rather than as independent variables. There should be an option to produce a 2-D plot of one dependent variable on the y-axis and another dependent variable on the x-axis, so that the publication plot can be reproduced. An example is Table 13 and Table 14 of https://www.hepdata.net/record/ins1731568 , cf. Figure 3 of the publication.