Add documentation on making faceted SPC Charts

[craig-shenton]

Add to docs on 'Visualising Data in R' page

• Docs: https://nhsengland.github.io/nhs-r-reporting/r-visualisation.html#nhs-r-plot-the-dots
• Code: https://github.com/nhsengland/nhs-r-reporting/blob/main/docs/r-visualisation.qmd

Example from Simon Wellesley-Miller:

library(NHSRdatasets)
library(NHSRplotthedots)
library(tidyverse)

df <- ae_attendances %>%
filter (org_code %in% c('RBN', 'REF', 'RWY', 'RGN'),
type == '1')

plot_obj <- ptd_spc(
df,
value_field = attendances,
date_field = period,
facet_field = org_code,
improvement_direction = "decrease"
)

plot <- ptd_create_ggplot(
plot_obj,
icons_position = 'none',
x_axis_date_format = "%b-%y"