Hi! I'm new to R and am working through the R for Data Science online book. I wanted to try do some data frames of my own and decided to graph some weather data. I would like to show a line graph of the temperature and the barometric pressure to show the relationship between changes in the two. However, the Y domains for the two series are different enough that you can't really see the trend (temps in F are 60-90 range and pressure in millibars are 1000+). I'm only interested in the trend, not the specific Y values. I tried adding a second axis, but that didn't show what I wanted and I also thought maybe if there was a way to do 2 graphs with the X data points aligned vertically it would accomplish the same goal. Does any have any suggestions on the best way to tackle this (again, I'm very new to R so its possible there is an easy solution I haven't read about yet)?

Thanks in advance!

Reprex:

```
library(ggplot2)
library(tidyr)
df <- data.frame(
ReadingDate = c("2019-08-19", "2019-08-20", "2019-08-21", "2019-08-22",
"2019-08-23"),
LowP = c(1013.85, 1013.2, 1012.89, 1013.7, 1015.79),
HighP = c(1016.53, 1016.73, 1015.86, 1015.45, 1019.11),
LowTemp = c(68.3, 68.54, 68.66, 69.49, 62.7),
HighTemp = c(86.89, 84.9, 81.43, 77.44, 70.27)
)
df %>%
gather(key, value, LowP, HighP, LowTemp, HighTemp) %>%
ggplot(aes(x=ReadingDate, y=value, color=key, group=key)) +
geom_line()
```

^{Created on 2019-08-24 by the reprex package (v0.3.0)}