How to use ggplot with Processing in QGIS 2.0?

Question

I'm trying to use R scripts within QGIS thanks to Processing. There are some example scripts available and I achieved to plots different columns of the attribute table (with the plot built-in package).

What I cannot do is using the ggplot package. This is one of the simple code I've written:

##Vector processing=group
##showplots
##Layer=vector
##Field1=Field Layer
##Field2=Field Layer
library("ggplot2")
ggplot(Layer)+
geom_point(aes([[Field1]], [[Field2]]))

But none plot is shown.

Log output says:

R execution commands
options("repos"="http://cran.at.r-project.org/")
tryCatch(find.package("ggplot2"), error=function(e) install.packages("ggplot2", dependencies=TRUE))
tryCatch(find.package("rgdal"), error=function(e) install.packages("rgdal", dependencies=TRUE))
tryCatch(find.package("raster"), error=function(e) install.packages("raster", dependencies=TRUE))
library("raster")
library("rgdal")
Layer = readOGR("/media/data/Universita/brema/gis/file_tesi/piezo_e_logger",layer="piezo_tutto_finale")
Field1="Br"
Field2="Ca"
png("/tmp/processing/f02d43598eb44b63aec89c02de38af54/RPLOTS.html.png")
library("ggplot2")
ggplot(Layer) + geom_point(aes(Layer[[Field1]], Layer[[Field2]]))

dev.off()

And that's a piece of the dbf table (trying to plot Cl vs Br):

ID                     Cl   Br
2818/22/0834    125.70  0.20
2818/22/0851    219.12  0.29
2818/22/0853    298.55  0.26
2818/22/0886    132.32  0.09
B0F                520.60   1.01
B12T           3435.49  3.83
B1T                2322.61  2.54
B21T           2800.90  3.18
B22F            366.87  0.64
B26T           1891.63  2.10
B33F            216.97  0.62
B4F                 383.98  0.44
P1                   82.01  0.14
P21                 365.56  0.74
P24                 199.98  0.73
P25                 308.71  0.90
P27                 217.58  0.58
P28                 436.09  0.32
P29                 711.77  1.08

Answer 1

So why doesn't it work?
Look at your code, specifically the line with the attribute fields. You need to point R/QGIS directly towards the field data vectors unless you attach them beforehand.

The following code works without issues for me (provided that you have installed ggplot2):

##Vector processing=group
##showplots
##Layer=vector
##Field1=Field Layer
##Field2=Field Layer
library(ggplot2)
ggplot(Layer) + geom_point(aes(Layer[[Field1]], Layer[[Field2]]))
# or save them beforehand into a vector: a <- Layer[[Field1]]

One example of a basic tutorial can be found on my blog.

EDIT: If you don't have the library installed, than the r-code will of course produce no output. Try to replace library(ggplot2) with

if (!require(ggplot2)){print("ggplot2 not installed. Will install");install.packages(ggplot2, dependencies = TRUE)}

Answer 2

I found out what was wrong. No Layer call inside ggplot, here the right code:

##Vector processing=group
##showplots
##Layer=vector
##a=Field Layer
##b=Field Layer
library("ggplot2")

ggplot() + 
geom_point(aes(Layer[[a]], Layer[[b]]))

Cheers