Hourly heatmaps for visualisation of air quality data

hourlyheatmap.R

## created after John Mackintosh's guide here http://johnmackintosh.com/2016-12-01-the-hourly-heatmap/. Thanks John!

library(ggplot2)
library(dplyr) # data wrangling 
library(viridis) # colour blind friendly palettes, see https://cran.r-project.org/web/packages/viridis/vignettes/intro-to-viridis.html
library(lubridate) # for easy date manipulation
library(ggExtra)
library(tidyr) 
library(readr)

airquality <- read_csv("~airquality.csv",   ## data downloaded from http://www.airqualityni.co.uk/data/download-data 'measured data' (i.e. hourly observations)
  col_types = cols(measure = col_number(),
  time = col_character()))

df <- airquality

## convert the date and time character values (e.g. '20/12/2016' & '01:00') to a datetime object
## then obtain the year, month, day and hour as individual variables for each observation
df<- df %>% mutate(newdate = parse_date_time(paste(date,time), "%d/%m/%Y H:%M")) %>%
  mutate(year = year(newdate),
                   month = month(newdate, label=TRUE),
                   day = day(newdate),
                   hour = hour(newdate))

#create plotting df
df <-df %>% select(day,hour,month,year,pollutant,measure,unit,site) %>% 
              filter(pollutant =='SO2' & !is.na(measure))
              ## ^^ change the pollutant to that which you want to measure (e.g. PM2.5, PM10, NO3, O3, SO2 etc.)

unit <- unique(df$unit)
pollutant <- unique(df$pollutant)

######## Plotting starts here#####################
p <-ggplot(df,aes(day,hour,fill=measure))+
  geom_tile(color= "white") + 
  scale_fill_viridis(direction=-1,name=paste("Hourly",pollutant,unit))
p <-p + facet_grid(site ~ month)
## ^^ facets charts by individual site and month. If we were to include multi-year data, would need to facet by this also
p <-p + scale_y_continuous(trans = "reverse", breaks = unique(df$hour))
p <-p + scale_x_continuous(breaks =c(1,10,20,31))
p <-p + theme_minimal(base_size = 8)
p <-p + labs(title=paste("Hourly",pollutant,"concentration"), x="Day", y="Hour Commencing")
p <-p + theme(legend.position = "bottom")+
  theme(plot.title=element_text(size = 14))+
  theme(axis.text.y=element_text(size=6)) +
  theme(strip.background = element_rect(colour="white"))+
  theme(plot.title=element_text(hjust=0))+
  theme(axis.ticks=element_blank())+
  theme(axis.text=element_text(size=7))+
  theme(legend.title=element_text(size=8))+
  theme(legend.text=element_text(size=6))+
  removeGrid()#ggExtra

p #draw chart

Hi Bob
When I run your script, the console returns the following error:

^^ facets charts by individual site and month. If we were to include multi-year data, would need to facet by this also

p <-p + scale_y_continuous(trans = "reverse", breaks = unique(df$hour))
Error: object 'p' not found
I'm afraid I do not understand what is going on here. Thanks