Plot order (#784)

* preserve order of distributions when plotting

* update news item

* add PR number and reviewer

Co-authored-by: James Azam <james.azam@lshtm.ac.uk>

---------

Co-authored-by: James Azam <james.azam@lshtm.ac.uk>

NEWS.md

@@ -36,6 +36,7 @@
 - a bug was fixed that caused delay option functions to report an error if only the CDF cutoff was specified. By @sbfnk.
 - a bug was fixed that led to the truncation PMF being shortened from the wrong side when the truncation PMF was longer than the supplied data. By @seabbs in #736 and reviewed by @sbfnk and @jamesmbaazam.
 - a bug was fixed that caused internal validation checks on delay distributions to fail if they contained non-parametric distributions. By @jamesmbaazam in #750 and reviewed by @seabbs.
+- a bug was fixed where combined distributions where shown in alphabetical order, rather than the order in which they were combined. By @sbfnk in #784 and reviewed by @jamesmbaazam.
 
 ## Documentation
 

R/dist_spec.R

@@ -692,9 +692,10 @@ plot.dist_spec <- function(x, samples = 50L, res = 1, cumulative = TRUE, ...) {
     }
     return(pmf_dt)
   })
-  pmf_data <- rbindlist(pmf_data)[,
-    type := factor("pmf", levels = c("pmf", "cmf"))
-  ]
+  pmf_data <- rbindlist(pmf_data)[, `:=`(
+    type = factor("pmf", levels = c("pmf", "cmf")),
+    distribution = factor(distribution, levels = unique(distribution))
+  )]
 
   # Plot PMF and CDF as facets in the same plot
   plot <- ggplot(