Customize label for an Interaction Plot

Suppose I have the following data which summarizes the values of a dependent variable (dv) at low and high vales of a predictor (x) and a moderator (z)

dfx <- data.frame(x = rep(c(-0.909, 0.909),2),
                  z = c(-0.676, -0.676, 0.676, 0.676),
                  dv = c(448.8, 464.4, 462.8, 472.1))

I would like to draw the dv as a function of the predictor and the moderator by customizing both the type of lines and the markers

library(ggplot2)
dfx$z <- as.factor(dfx$z) # turn the moderator as a factor

ggplot(dfx, aes(x=x, y = dv, colour=z, shape=z))+
  geom_line()+
  geom_point()+
  scale_y_continuous("dv", limits = c(440,480))+
  theme_classic()+
  scale_colour_manual  ("A title", values=c("black", "black"), labels=c("-1SD", "+1SD"))+
  scale_shape_manual   ("A title", values=c(2,15),labels=c("-1SD","+1SD"))

However, I would like to customize the linetype as well.

When I add:

ggplot(dfx, aes(x=x, y = dv, colour=z, shape=z, linetype=z)

the following plot emerges. Yet I cannot either hide the second label or customize it through scale_linetype_manual

Is there a way to fix this? I searched the community but could not find any method to overcome this problem. Also, when I suppress the second label, the legend key does not show the line type and mark. Any hint will be very much appreciated.

From your description, I'm guessing you want something like this:

library(ggplot2)

ggplot(dfx, aes(x=x, y = dv, shape=z, linetype=z)) +
  geom_line()+
  geom_point()+
  scale_y_continuous("dv", limits = c(440,480))+
  theme_classic()+
  scale_linetype_manual("A title",
                      values=c(1, 3),
                      labels=c("-1SD", "+1SD")) +
  scale_shape_manual("A title",
                     values=c(2,15),
                     labels=c("-1SD","+1SD"))