The github site for the package has these installation instructions:
To install the smwrGraphs package, first install the "remotes" package. Then, using the remotes package, install "smwrData", "smwrBase", and finally "smwrGraphs":
remotes::install_github("USGS-R/smwrData")
remotes::install_github("USGS-R/smwrBase")
remotes::install_github("USGS-R/smwrGraphs")