使用 ggplot2 如何在绘制线条后用 aes() 点 plot ？

Question

I'm using ggplot2 to show lines and points on a plot. What I am trying to do is to have the lines all the same color, and then to show the points colored by an attribute. My code is as follows:

# Data frame
dfDemo <- structure(list(Y = c(0.906231077471568, 0.569073561538186, 
0.0783433165521566, 0.724580209473378, 0.359136092118470, 0.871301974471722, 
0.400628333618918, 1.41778205350433, 0.932081770977729, 0.198188442350644
), X = c(0.208755495088456, 0.147750173706688, 0.0205864576474412, 
0.162635017485883, 0.118877260137735, 0.186538613831806, 0.137831912094464, 
0.293293029083812, 0.219247919537514, 0.0323148791663826), Z = c(11112951L, 
11713300L, 14331476L, 11539301L, 12233602L, 15764099L, 10191778L, 
12070774L, 11836422L, 15148685L)), .Names = c("Y", "X", "Z"
), row.names = c(NA, 10L), class = "data.frame")

# Variables
X = array(0.1,100)
Y = seq(length=100, from=0, by=0.01)

# make data frame
dfAll <- data.frame()

# make data frames using loop
for (x in c(1:10)){

    # spacemate calc
    Floors = array(x,100)

    # include label
    Label = paste(' ', toString(x), sep="") 
    df1 <- data.frame(X = X * x, Y = Y, Label)

    # merge df1 to cumulative df, dfAll
    dfAll <- rbind(dfAll, df1)

}

# plot 
pl <- ggplot(dfAll, aes(x = X, y = Y, group = Label, colour = 'Measures')) + geom_line() 

# add  points to plot
pl + geom_point(data=dfDemo, aes(x = X, y = Y)) + opts(legend.position = "none")

This almost works, but I am unable to color the points by Z when I do this. I can plot the points separately, colored by Z using the following code:

ggplot(dfDemo, aes(x = X, y = Y, colour = Z)) + geom_point()

However, if I use the similar code after plotting the lines:

pl + geom_point(data=dfDemo, aes(x = X, y = Y, colour = Z)) + opts(legend.position = "none")

I get the following error:

Error: Continuous variable () supplied to discrete scale_hue.

I don't understand how to add the points to the chart so that I can colour them by a value. I appreciate any suggestion how to solve this.

Answer 1

The issue is that they are colliding the two colour scales, one from the ggplot call and the other from geom_point. If you want the lines of one colour and the points of different colours then you need to erase the colour setting from ggplot call and put it inside the geom_line outside the aes call so it isn't mapped. Use I() to define the colour otherwise it will think is just a variable.

    pl <- ggplot(dfAll, aes(x = X, y = Y, group = Label)) + 
                 geom_line(colour = I("red"))
    pl + geom_point(data=dfDemo, aes(x = X, y = Y, colour = Z)) + 
                    opts(legend.position = "none")

HTH