R: facet_grid plot of differences between groups using ggplot2 -

i'm trying create series of plots showing differences between groups of measured variable, , searching efficient way using facet_grid feature of ggplot2 in r.

here illustrative example:

# sample input data df <- data.frame(year=rep(c(2011:2015), 2),                   value=c(0:4, 1:5),                  scenario=rep(c("a","b"), each=5))  # make sample plot p <-    ggplot(df, aes(x=year, y=value)) +   geom_point() + geom_line() +   facet_grid(scenario ~ scenario) 

this produces following sample plot, in value plotted against year separately each scenario combination:

(i assume second row not plotted because identical first).

however, looking plot where, in each facet, (value in scenario on top) - (value in scenario on right) plotted year. specifically:

• upper left plot (value a) - (value a) = 0 years.
• upper right plot (value b) - (value a) = 1 years.
• lower left plot (value a) - (value b) = -1 years.
• lower right plot (value b) - (value b) = 0 years

i have not been able find built-in or automated difference command facet_grid. initial thought pass function y argument ggplot, given data frame has single value column got stumped. guessing there might solution using combination of dplyr , reshape2 cannot wrap head around how implement it.

here option using functions tidyr first spread data allow contrasts calculated, gathering allow plotting:

forplotting <-   df %>%   spread(scenario, value) %>%   mutate(`a - b` = - b          , `b - a` = b -          , `a - a` = 0          , `b - b` = 0) %>%   gather(comparison, difference, -(year:b) ) %>%   separate(comparison, c("first val", "second val"), " - ") 

that returns data.frame (just head here):

  year b first val second val difference 1 2011 0 1                  b         -1 2 2012 1 2                  b         -1 3 2013 2 3                  b         -1 4 2014 3 4                  b         -1 5 2015 4 5                  b         -1 6 2011 0 1         b                   1 

and can plot so:

ggplot(forplotting        , aes(x = year, y = difference)) +   geom_point() + geom_line() +   facet_grid(`first val` ~ `second val`) 

the bigger question why want this. assume know plotting 2 sets different color lines easier visualization:

ggplot(df, aes(x=year, y=value, col = scenario)) +   geom_point() + geom_line() 

so, assuming have more complicated data -- specifically, lots more columns compare. so, here approach automate (and simplify) many of above steps multiple columns. approach same, uses mutate_ allow pass in vector columns trying create.

df <-   data.frame(     year = 2011:2015     , = 0:4     , b = 1:5     , c = 2:6     , d = 3:7   )  allcontrasts <-   outer(colnames(df)[-1]         , colnames(df)[-1]         , paste         , sep = " - ") %>%   as.character() %>%   setnames(., .) %>%   as.list()  forplotting <-   df %>%   mutate_(.dots = allcontrasts) %>%   select(-(a:d)) %>%   gather(comparison, difference, -year ) %>%   separate(comparison, c("first val", "second val"), " - ") %>%   filter(`first val` != `second val`)  ggplot(forplotting        , aes(x = year, y = difference)) +   geom_point() + geom_line() +   facet_grid(`first val` ~ `second val`) +   theme(axis.text.x = element_text(angle = 90)) 

gives this:

why can not leave alone? playing standard evaluation much. if have non-parsing column names (e.g, things spaces) above fail. so, here example such column names, showing addition of backticks ensure columns parse correctly.

df <-   data.frame(     year = 2011:2015     , value = c(0:4, 1:5, 2:6, 3:7)     , scenario = rep(c("unit 1", "exam 2"                        , "homework", "final exam")                      , each = 5)   ) %>%   spread(scenario, value)  allcontrasts <-   outer(paste0("`", colnames(df)[-1], "`")         , paste0("`", colnames(df)[-1], "`")         , paste         , sep = " - ") %>%   as.character() %>%   setnames(., .) %>%   as.list()  forplotting <-   df %>%   mutate_(.dots = allcontrasts) %>%   select_(.dots = paste0("-`", colnames(df)[-1], "`")) %>%   gather(comparison, difference, -year ) %>%   separate(comparison, c("first val", "second val"), " - ") %>%   filter(`first val` != `second val`) %>%   mutate_each(funs(gsub("`", "", .)), `first val`, `second val`)  ggplot(forplotting        , aes(x = year, y = difference)) +   geom_point() + geom_line() +   facet_grid(`first val` ~ `second val`) +   theme(axis.text.x = element_text(angle = 90))