library(dplyr)
library(qgam)
library(mgcv)
data(UKload)
test <- gam(
  NetDemand ~ te(wM, Posan, by = Year),
  data = UKload %>% mutate(Year = as.factor(Year))
)

library(visreg)
test2 <- gam(
  NetDemand ~ s(wM, by = Year),
  data = UKload %>% mutate(Year = as.factor(Year))
)
visreg(test2, xvar = "wM", by = "Year")

visreg2d(test, xvar = "wM", yvar = "Posan", by = "Year")

vis.gam(test, plot.type = "contour", cond = list(Year = 2011))