Quantitative analysis of non-standard scatter plots.

Hi,

I am having difficulties in finding ways to analyse scatter plots and
quantitatively differentiate between them. Since this is best demonstrated
by a figure, I have attached one with 4 subfigures (a)-(d). Qualitatively
(and I hope you would agree with me), sub-figures (a), (b), and (d) seem to
represent uniform 2d scatters. It is hard for me to quantify it, but it
seems as if the envelopes (max values of y-axis) are independent of the
values of the x-axis. Sub-figure (c), on the other hand, exhibits a
different behaviour, as the envelope seems to expand as the x-values
increase (at least till some threshold, and with the exception of a single
outlier toward the begining of the x-axis). My question is how can I verify
the above observation quantitatively, and does R provide the required tools
for doing so? I have tried to perform linear fittings and to claim that the
slope of the fitted line in (c) is more significant than the lines in (a),
(b), and (d), but this does not seem convincing to me. Are there standard
toolboxes for such cases?

Thanks for any suggestions / tips,
Firas.


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