Create `health-plots.R` that includes various plotting functions

[SaranjeetKaur]

Changes made:

• Instead of the separate plotting files, namely, risk-factors.R, rf-diff.R, inc-diff.R, burden.R, and inc-cum-diff.R, there is a single file called health-plots.R.
• health-plots.R contains several plotting functions: riskfactors(), riskfactors_diff(), inc_diff(), inc_cum(), burden_disease(), life_exp().
• To create plot for different metrics, select appropriate option in the function. No need to write the same function repeatedly for each metric.

To be discussed with Jack and Jingmin:

• [ ] inc_cum have various values for element_text(size = 10) namely 9.5/10/12. Set 10 as default
• [ ] riskfactors has scale_y_continuous(labels = scaleFUN) as default, whereas it also uses scale_y_continuous(labels = scales::comma) occasionally
• [ ] dalydiffcum has color = "purple" but in burden_disease default is set to colour = "#FF1493"
• [ ] life_exp() is a separate function from burden_disease()
• [ ] arguments like colour, linewidth for the geom_line, breaks, time, etc are hard-coded: it might help to provide them as arguments in the function, in case the user wants to change the values.
• [ ] check if the values in the tests are valid or not

To-do:

• [ ] Include examples in the roxygen comments of each function (`#' @examples)

[SaranjeetKaur]

Seems sensible to me!

Do we want to tag @jzhu20 as a reviewer? Up to you (and I guess you haven't done a PR before, @jzhu20?), but might be useful for these types of PRs.

Thanks @alexdewar! Yes, I have tagged @jzhu20 as a reviewer on this one. It would be helpful to have her inputs here.

[alexdewar]

Oh, I didn't spot that you'd already done it :facepalm: