Inconsistent behavior of geom_line and geom_smooth wrt. color

[aecay]

geom_smooth does not pick up its color from a scale_color_manual unless color is explicitly added to the aesthetic; geom_line does. Perhaps this is best explained by illustration:

foo <- data.frame(x = rnorm(30),  y = rnorm(30),  z = 1:3)
foo$z <-  factor(foo$z)
# the below graph has blue smooth lines
ggplot(foo,  aes(x = x,  y = y, linetype=z)) + geom_smooth() + 
  scale_color_manual(values = c("black"))
# the below graph has black lines
ggplot(foo,  aes(x = x,  y = y, linetype=z)) + geom_line() + 
  scale_color_manual(values = c("black"))
# this makes the first line black, but the other two don't show up
ggplot(foo,  aes(x = x,  y = y, linetype=z, color = z)) + geom_smooth() + 
  scale_color_manual(values = c("black"))

It seems like a bug for the two functions to behave differently, but I can't tell which one is misbehaving. I would prefer if they both behaved like geom_line, because I am trying to "zero out" all the colors in a graph with the scale_color_manual (for a dead-tree version of a graph). If there is an alternative, better way to do that, I would also be interested in hearing about it...

Thanks.

[hadley]

I'm still not sure what's causing the bug, but if you just want black lines, do geom_line(colour = "black")

[wch]

Is this a bug? I think it might be expected behavior.

In this case, the color in scale_color_manual shouldn't set the color of the line. This is because no variable is mapped to color -- therefore doing stuff to the color scale has no effect. geom_smooth and geom_line both behave the same way -- it's just that the default color of geom_smooth is blue and the default color of geom_line is black:

foo <- data.frame(x = rnorm(30),  y = rnorm(30),  z = 1:3)
foo$z <-  factor(foo$z)
# Setting color in scale has no effect
ggplot(foo,  aes(x = x,  y = y, linetype=z)) + geom_smooth() + 
  scale_colour_manual(values = c("red"))
ggplot(foo,  aes(x = x,  y = y, linetype=z)) + geom_line() + 
  scale_colour_manual(values = c("red"))

# Setting it directly works
ggplot(foo,  aes(x = x,  y = y, linetype=z)) + geom_smooth(colour="red")
ggplot(foo,  aes(x = x,  y = y, linetype=z)) + geom_line(colour="red")

As for the issue, regarding why the other lines don't show up when colour is mapped and the scale only has one value, I think this is happening because the other values are assumed to be NA if they're not specified.

# Only one line shows up, in black
ggplot(foo,  aes(x = x,  y = y, linetype=z, colour = z)) + geom_smooth() + 
  scale_colour_manual(values = c("black"))

# First and third lines show up
ggplot(foo,  aes(x = x,  y = y, linetype=z, color = z)) + geom_smooth() + 
  scale_color_manual(values = c("black",NA,"red"))

I think it would probably be a good idea to print a message when not enough values are specified in a discrete scale.

Update: I checked 0.8.9, and the behavior is the same.

[hadley]

Agreed, this is expected behaviour. I'm not sure if a warning would be the right thing to do:

How would that differ from the following bevahiour?

ggplot(foo,  aes(x = x,  y = y, linetype=z, color = z)) + 
  geom_smooth() + 
  scale_colour_manual(values = "black", na.value = "red")

But maybe that behaviour is undesirable?

[wch]

I have a hard time envisioning situations where using na.value this way would be desirable... when would you want to pass a vector with NAs to values and then tell it to substitute the NAs with something else?

I guess the NA substitution makes sense for continuous color scales, but in those cases, the NAs are calculated from the scale -- they're not manually specified, as is the case here.

[hadley]

Good point. I'll make the fix.

[aecay]

Hrm. I have lost track somewhat of the discussion. I think I understand why the geom_smooth is blue whereas the geom_line is black.

From the point of view of a user, it would be convenient if a quick print-suitable version of a graph could be generated by last_plot() + theme_bw() + <something>. That is what I was trying to use the scale_color_manual to do -- to remove all the color from the plot. So, I think that the missing piece is one or both of:

1. making geom_smooth and friends read their default color from the theme, instead of having a global default. This would let them be black in theme_bw, but blue in theme_default
2. adding a scale_color_constant function, which would override any color that is specified by an aesthetic with a single value (in this case black). This has the advantage of avoiding the need to specify black in each geom that makes up the graph.

That turns this from a bug report into a feature request, I suppose. I've looked at the code involved, and while I wouldn't say that I have wrapped my head around all its object-orientedness, I could take a crack at implementing these things if there is interest in them.

[hadley]

The manual scales issue is now #512.

Currently, there's no way to change the default aesthetics of geoms on mass. This may be something that will be possible after the layer rewrite. Could you please file that as a separate issue, outlining your use case?

Thanks!