secaxis = sec_axis ~rescalec005

This line of code is using the sec_axis() function in R's ggplot2 package to create a secondary axis for plotting.

The sec_axis() function takes two arguments: trans and name.

In this specific code, the trans argument is ~rescale(.,c(0,0.5)). This means that the values on the secondary axis will be transformed using the rescale() function, which scales the values to a range between 0 and 0.5.

The name argument is not specified in this code snippet, but it would typically be used to provide a label for the secondary axis.