1. Plotting and mapping signals

Choropleth maps

The default plot method for covidcast_signal objects, plot.covidcast_signal(), produces choropleth maps by using ggplot2 and the usmap package:

plot(comb)

The color scheme is automatically chosen to be similar to that used on the online COVIDcast mapping tool. Also, by default, this map shows the most recent day of data available in the data frame. One can choose the day and also choose the color scales, transparency level for mega counties, and title:

plot(comb, time_value = "2020-07-04", choro_col = cm.colors(10), alpha = 0.4,
     title = "Combination of COVID-19 indicators on 2020-07-04")

By providing breaks and colors, we can create custom color scales, for example to have a log-spaced color scale for incident case counts:

breaks <- c(0, 1, 2, 5, 10, 20, 50, 100, 200)
colors <- c("#D3D3D3", "#FFFFCC", "#FEDDA2", "#FDBB79", "#FD9950", "#EB7538",
            "#C74E32", "#A3272C", "#800026")

# Note that length(breaks) == length(colors) by design. This works as follows:
# we assign colors[i] iff the value satisfies breaks[i] <= value < breaks[i+1],
# where we take breaks[0] = -Inf and breaks[N+1] = Inf, for N = length(breaks)

plot(inum, choro_col = colors, choro_params = list(breaks = breaks),
     title = "New COVID cases (7-day trailing average) on 2020-07-14")

Lastly, we show how we can use custom breaks to (visually) answer the question: which counties have cumulative case rates of at least 1/100?

cprop <- suppressMessages(
  covidcast_signal(data_source = "usa-facts",
                   signal = "confirmed_cumulative_prop",
                   start_day = "2020-07-01", end_day = "2020-07-14")
)

breaks <- c(0, 1000)
colors <- c("#D3D3D3", "#FFC0CB")

plot(cprop, choro_col = colors,
     choro_params = list(breaks = breaks, legend_width = 3),
     title = "Cumulative COVID cases per 100k people on 2020-07-14")

Bubble maps

As an alternative to choropleth maps, we can also quickly plot bubble maps. By default, bubble maps have 8 bubble size bins evenly spaced over the range, where zero always means zero bubble size. The legend shows all bins, interpreted as each bubble size meaning at least the corresponding value.

plot(inum, plot_type = "bubble")

## Warning in plot_bubble(x, time_value = time_value, include = include, range
## = range, : Bubble maps can be hard to read when there is missing data;the
## locations without data are filled in gray.

As before, we can of course set customized breaks. As values to the left of the first bin do not get drawn, this map is much sparser, and highlights areas with larger case counts.

plot(inum, plot_type = "bubble",
     bubble_params = list(breaks = seq(20, 200, len = 6)))

## Warning in plot_bubble(x, time_value = time_value, include = include, range
## = range, : Bubble maps can be hard to read when there is missing data;the
## locations without data are filled in gray.

As a final example, suppose we want to plot only counties in the state of Texas. We’d like to compare counts per 100,000 against absolute counts, so we fetch the proportion signal:

iprop <- suppressMessages(
  covidcast_signal(data_source = "usa-facts",
                   signal = "confirmed_7dav_incidence_prop",
                   start_day = "2020-07-01", end_day = "2020-07-14")
)

Then we make two maps side-by-side with custom ranges:

library(gridExtra)

## 
## Attaching package: 'gridExtra'

## The following object is masked from 'package:dplyr':
## 
##     combine

breaks1 <- c(1, 10, 100, 1000)
breaks2 <- c(10, 50, 100, 500)

p1 <- plot(inum, plot_type = "bubble",
           bubble_params = list(breaks = breaks1, max_size = 6),
           include = "TX", bubble_col = "red",
           title = paste("Incidence number on", max(inum$time_value)))
p2 <- plot(iprop, plot_type = "bubble",
           bubble_params = list(breaks = breaks2, max_size = 6),
           include = "TX", bubble_col = "red",
           title = paste("Incidence rate on", max(iprop$time_value)))

grid.arrange(p1, p2, nrow = 1)

Time series plots

Let’s fetch the combination indicator and case counts, but for all states rather than for all counties. This will make the time series plots more manageable.

suppressMessages({
comb_st <- covidcast_signal(data_source = "indicator-combination",
                               signal = "nmf_day_doc_fbc_fbs_ght",
                               start_day = "2020-04-15", end_day = "2020-07-01",
                               geo_type = "state")
inum_st <- covidcast_signal(data_source = "usa-facts",
                               signal = "confirmed_7dav_incidence_num",
                               start_day = "2020-04-15", end_day = "2020-07-01",
                               geo_type = "state")
})

By default, time series plots show all available data, including all geographies. A line for every state would be unmanageable, so let’s select a few states and plot all data for them:

library(dplyr)

states <- c("ca", "pa", "tx", "ny")
plot(comb_st %>% filter(geo_value %in% states), plot_type = "line")

plot(inum_st %>% filter(geo_value %in% states), plot_type = "line")

Notice how in Texas, the combined indicator rose several weeks in advance of confirmed cases, suggesting the signal could be predictive. Delphi is investigating these signals for their usefulness in forecasting, as well as hotspot detection and will publish results when they are available.

Manual plotting

Using ggplot2 or your favorite plotting package, we can easily plot time series manually, without using the plot.covidcast_signal() method. You can use this to customize the appearance of your plots however you choose.

For example:

library(ggplot2)

suppressMessages({
comb_md <- covidcast_signal(data_source = "indicator-combination",
                               signal = "nmf_day_doc_fbc_fbs_ght",
                               start_day = "2020-06-01", end_day = "2020-07-15",
                               geo_values = name_to_fips("Miami-Dade"))
inum_md <- covidcast_signal(data_source = "usa-facts",
                               signal = "confirmed_7dav_incidence_num",
                               start_day = "2020-06-01", end_day = "2020-07-15",
                               geo_values = name_to_fips("Miami-Dade"))
})

# Compute the ranges of the two signals
range1 <- inum_md %>% select("value") %>% range
range2 <- comb_md %>% select("value") %>% range

# Function to transform from one range to another
trans <- function(x, from_range, to_range) {
  (x - from_range[1]) / (from_range[2] - from_range[1]) *
    (to_range[2] - to_range[1]) + to_range[1]
}

# Convenience functions for our two signal ranges
trans12 <- function(x) trans(x, range1, range2)
trans21 <- function(x) trans(x, range2, range1)

# Transform the combined signal to the incidence range, then stack
# these rowwise into one data frame
df <- select(rbind(comb_md %>% mutate_at("value", trans21),
                   inum_md), c("time_value", "value"))
df$signal <- c(rep("Combined indicator", nrow(comb_md)),
               rep("New COVID-19 cases", nrow(inum_md)))

# Finally, plot both signals
ggplot(df, aes(x = time_value, y = value)) +
  labs(x = "Date", title = "Miami-Dade County") +
  geom_line(aes(color = signal)) +
  scale_y_continuous(
    name = "New COVID-19 cases (7-day trailing average)",
    sec.axis = sec_axis(trans12, name = "Combination of COVID-19 indicators")
  ) +
  theme(legend.position = "bottom",
        legend.title = ggplot2::element_blank())

Again, we see that the combined indicator starts rising several days before the new COVID-19 cases do, an exciting phenomenon that Delphi is studying now.