Update plotProportions.R

[ignatiusm]

There have been a couple of changes that meant that the plotProportions function was no longer working. I updated the toMatrix function to handle if objects have multiple classes (e.g. c("data.frame", "tbl_df") ), and also incorporated a change from ggplot (https://ggplot2.tidyverse.org/news/index.html#ggplot2-334) to resolve a warning message.