adding ppc_residual_scatter

[behramulukir]

In #361 and #358, as part of our discussion related to enhancing error/residual plots in general, we talked about implementing a new function named ppc_residual_scatter. The idea of such a function originated in #349 and #350 due to a demand for a function that creates plots of y - stat(yrep) instead of what existing error functions plot, stat(y - yrep). So, the logic of ppc_residual_scatter is what @jgabry said in his comment here:

2. Keep ppc_error_scatter_avg as you have it and add a new one that does y - stat(y_rep) on the y-axis and stat(y_rep) on the x-axis. We could call it ppc_residual() or something like that. I think with the right documentation having both would probably be ok.

This PR is about my work on creating the ppc_residual_scatter.

• implement ppc_residual_scatter
• write documentation
• add tests

The problem I have is that I am a little confused about my implementation. When I look at it, it seems logical and in some cases, I am fine with the results. However, there are some cases which confused me, such as ppc_residual_scatter(y, yrep, x) and ppc_error_scatter_avg(y, yrep, x) having the same plot. I think these plots shouldn't look identical, but somehow, to me, they look like they are the same plot.

Examples

y <- example_y_data()
yrep <- example_yrep_draws()
x <- example_x_data()

ppc_residual_scatter(y, yrep)

ppc_error_scatter_avg(y, yrep)

ppc_residual_scatter(y, yrep, x)

ppc_error_scatter_avg(y, yrep, x)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.05%. Comparing base (e7d10d9) to head (015e4d3).

Files with missing lines	Patch %	Lines
R/ppc-errors.R	0.00%	33 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #368      +/-   ##
==========================================
- Coverage   98.62%   98.05%   -0.57%     
==========================================
  Files          35       35              
  Lines        5729     5762      +33     
==========================================
  Hits         5650     5650              
- Misses         79      112      +33     

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