How to insert caption in graph with two axes and in r?

Because the bars use fill and the rows / dots use color , you can specify separate scales for each. Enter the name you want in the legend within the respective aesthetics and colors in the manual scale options. In this way, the legend keys will match the type of geometry you are using.

ggplot(obs, aes(x = Mes)) +
  geom_bar(aes(y = Acid, fill="bruto"), stat = "identity", alpha=0.4) +
  geom_point(aes(y = Obt*14, color="líquido"), size=2, group = 1) +
  geom_line(aes(y = Obt*14, color="líquido"), size=1, group = 1) +
  scale_y_continuous(sec.axis = sec_axis(~./14, name = "Total líquido")) +
  scale_fill_manual("Totais:      ", values = "darkblue") +
  scale_color_manual(NULL, values = "#a50026") +
  labs(y = "Total bruto", x = "Mês") +
  theme(legend.position = "bottom") +
  guides(fill = guide_legend(order = 1), color = guide_legend(order = 2))

I think the code below solves the problem in a satisfactory way, at least according to my criteria.

ggplot(obs, aes(x = Mes)) +
  geom_bar(aes(y = Acid, color="Bruto"), fill="darkblue", stat = "identity", 
    alpha=0.4) +
  geom_point(aes(y = Obt*14), color="#a50026", size=2, group = 1) +
  geom_line(aes(y = Obt*14, color="Líquido") , size=1, group = 1, linetype=1) +
  scale_y_continuous(sec.axis = sec_axis(~./14, name = "Total líquido")) +
  labs(y = "Total bruto", x = "Mês") +
  scale_color_manual(name="Totais", values=c("#9c9ec5", "#a50026"), 
    guide = guide_legend(override.aes=aes(fill=NA)))

The key is to map the colors of the bars and the line within their respective aes . Note that I have already called these colors Gross and Liquid , respectively. Oh, and I set a color for the contour of the bar, because on the initial chart only the fill was set (not that this is a problem).

Next, you just created a scale_color_manual , set the caption name and color values. The problem I encountered was setting the blue color correctly. Originally a darkblue , but since a alpha=0.4 was applied, the color we perceive is a little clearer. So, I had to detect exactly what the blue tone of the bar was with the alpha applied. In this case, it gave # 9c9ec5.

Unfortunately, I do not know a way to combine darkblue (or any other color whatsoever) with alpha in the caption automatically to generate the desired color.