Issue with ggplot2 line plot: Tilted straight lines instead of expected line plot

I'm trying to create a line plot using ggplot2 in R, but I'm facing an issue where the plot shows tilted straight lines instead of the expected line plot as shown in the figure below.

Here's the code I'm using:

aws_plot <- df_aws_atm %>% ggplot(aes(x=Month, y=values, color=variables, 
group=1)) +
geom_line(linewidth=0.5) +
scale_color_manual(values = c("P"="black","E"="blue","C"="green",
"WVSC"="red")) +
# scale_y_continuous(limits = c(-50, 1500)) +
scale_x_discrete(expand = c(0.02, 0)) +
ggtitle("Atmospheric Water Balance") +
xlab("Month") +
ylab("mm") +
theme_bw()

And here's the data I'm using:

> dput(df_aws_atm)
structure(list(Month = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 
2L, 3L, 3L, 3L, 3L, 4L, 4L, 4L, 4L, 5L, 5L, 5L, 5L, 6L, 6L, 6L, 
6L, 7L, 7L, 7L, 7L, 8L, 8L, 8L, 8L, 9L, 9L, 9L, 9L, 10L, 10L, 
10L, 10L, 11L, 11L, 11L, 11L, 12L, 12L, 12L, 12L), levels = c("Jan", 
"Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", 
"Nov", "Dec"), class = "factor"), variables = structure(c(1L, 
2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 
2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 
2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L), levels = c("P", 
"E", "C", "WVSC"), class = "factor"), values = c(61.6012042725641, 
19.4143472506923, 41.107596204, -1.07926081812821, 61.0211111110256, 
-5.24147855594872, 66.7437418039231, 0.48115213725641, 80.6383290582051, 
7.16041454266667, 76.3176395729744, 2.83972504705128, 100.314478631795, 
69.3612280245385, 34.7814502373077, 3.82819962705128, 112.722914529231, 
92.58926418, 27.2810184358718, 7.14736808612821, 105.177636751538, 
110.168340093846, 2.30605669376923, 7.2967600354359, 99.9483119687179, 
126.626434667949, -22.3589573936923, 4.31916530276923, 87.8823653871795, 
112.96183799, -26.497900053, -1.41842745128205, 80.9593547017949, 
79.0862466384615, -5.19606537007692, -7.06917343351282, 84.362542735641, 
48.421970115641, 28.3443924478205, -7.5961801698718, 82.4711452974359, 
13.4414487457436, 63.4596880341795, -5.57000852123077, 70.4012179492308, 
8.49156181276923, 58.8562424521282, -3.05341368164103)), row.names = c(NA, 
-48L), class = c("tbl_df", "tbl", "data.frame"))
> 

library(ggplot2)

df_aws_atm  |> 
  ggplot(aes(x=Month, y=values, colour=variables, group = variables)) +
  geom_line(linewidth=0.5) +
  scale_color_manual(values = c("P"="black","E"="blue","C"="green",
                                "WVSC"="red")) +
  # scale_y_continuous(limits = c(-50, 1500)) +
  scale_x_discrete(expand = c(0.02, 0)) +
  ggtitle("Atmospheric Water Balance") +
  xlab("Month") +
  ylab("mm") +
  theme_bw()

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^{Created on 2023-07-03 with reprex v2.0.2}