GeomTimeline: Create the ggproto object for geom_timeline
In jmmark/QuakeCoursera: Exploring the NOAA Significant Earthquake Database

This function creates the ggproto Geom object to plot the necessary grid grob for geom_timeline

1	GeomTimeline

An object of class GeomTimeline (inherits from Geom, ggproto) of length 5.

data('clean_NOAA')
NOAA <- clean_NOAA
plot_NOAA <- NOAA[NOAA$COUNTRY=="USA" | NOAA$COUNTRY=="CANADA",]
xmin <- lubridate::ymd("2010-01-01")
xmax <- lubridate::ymd("2017-07-01")
plt <- ggplot2::ggplot(data = plot_NOAA, ggplot2::aes(x = DATE, y = COUNTRY)) +
  geom_timeline(ggplot2::aes(size = EQ_PRIMARY), x_min = xmin, x_max = xmax) +
  ggplot2::theme_minimal()
print(plt)