Average variables and create line graph

General
2

Hi, welcome!

Is this what you are trying to do?

# Library calls
library(tidyverse)

# Sample data on a copy/paste friendly format
mydata <- data.frame(
  stringsAsFactors = FALSE,
          Cultivar = c("A","A","B","B","C","C",
                       "A","A","B","B","C","C","A","A","B","B","C","C"),
            Period = c(1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3),
            Height = c(32,23,34,43,43,43,24,43,
                       32,23,33,34,23,33,44,32,42,42)
)

# Relevant code
mydata %>% 
    group_by(Period,Cultivar) %>% 
    summarise(mean_heigh = mean(Height, na.rm = TRUE)) %>% 
    ggplot(aes(x = Period, y = mean_heigh, color = Cultivar)) +
    geom_line()

Created on 2020-02-13 by the reprex package (v0.3.0.9001)

If this doesn't solve your problem, please provide a proper REPRoducible EXample (reprex) illustrating your issue.