Plotting contour plots from melted dataframes - r

Question

I have a dataset which is a function of time and frequency. When I melt it I want to retain actual time (date) and frequency values as I want a 2d plot with y axis as frequency and x axis as time.
I tried to retain a column of desired axis values but melting makes it factor and stat_contour throws error.
My data is some thing like the following
a = data.frame(date=time,power=power)
names(a) = c('date',period)
where period is
[1] 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8
[23] 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8
[45] 8 8 8 8 8 8 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16
[67] 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16
[89] 16 16 16 16 16 16 16 16 16 16 16 16 32 32 32 32 32 32 32 32 32 32
[111] 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32
[133] 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 64 64 64 64
[155] 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64
[177] 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64 64
[199] 64 64 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128
[221] 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128
[243] 128 128 128 128 128 128 128 128 256
power = melt(a,id.vars = 'date')
date period power
1 850-01-01 8 0.05106766
2 851-01-01 8 0.05926821
3 852-01-01 8 0.06783015
4 853-01-01 8 0.07681627
5 854-01-01 8 0.08636516
6 855-01-01 8 0.09667054
ggplot(power, aes(x = date, y = period, z = power)) +
stat_contour()
this gives an error as period column is a factor; if I make it numeric I loose the exact Y axis labels. Is there any workaround?
thanks

Answer 1

You did not provide a reproducible example, but in principle you have to change the axis labels manually:
library(ggplot2)
d <- expand.grid(x = 1:100, y = paste("P", 1:100))
d$z <- rnorm(10000)
ggplot(d, aes(x, as.numeric(y), z = z)) +
geom_contour() +
scale_y_continuous(breaks = seq_along(levels(d$y)), breaks = levels(d$y))
However, I find it a bit difficult to really understand what a contour plot with one axis being a qualitative should represent. My guess is that your y-axis is a factor because of the melt operation and that there is some column wrongly specified as factor and hence the overall column is a factor when in fact it should be a a numeric (that is it shows 8, should represent 8 but is actually a factor with level 8).
If this is the case, a normal as.numeric does not work as expected b/c it maps simply the first level to 1 the second level to 2 and so on. But you want it to be mapped to the numeric of the string representation. In this case something like this should work:
library(ggplot2)
d <- expand.grid(x = 1:100, y = sample(1000, 100)) # y should be a numeric but is a factor
d$z <- rnorm(10000)
ggplot(d, aes(x, as.numeric(as.character(y)), z = z)) +
geom_contour()
This all is, however, guesswork, because you did not provide us with a full reproducible example. Please read and follow these gudieline in the future when posting questions here how to ask a good question and how to give a reproducible example