Ordering the axis labels in geom_tile

Question

I have a data frame containing order data for each of 20+ products from each of 20+ countries. I have put it in a highlight table using ggplot2 with code similar to this:

require(ggplot2)
require(reshape)
require(scales)

mydf <- data.frame(industry = c('all industries','steel','cars'), 
    'all regions' = c(250,150,100), americas = c(150,90,60), 
     europe = c(150,60,40), check.names = FALSE)
mydf

mymelt <- melt(mydf, id.var = c('industry'))
mymelt

ggplot(mymelt, aes(x = industry, y = variable, fill = value)) +
    geom_tile() + geom_text(aes(fill = mymelt$value, label = mymelt$value))

Which produces a plot like this:

In the real plot, the 450 cell table very nicely shows the 'hotspots' where orders are concentrated. The last refinement I want to implement is to arrange the items on both the x-axis and y-axis in alphabetical order. So in the plot above, the y-axis (variable) would be ordered as all regions, americas, then europe and the x-axis (industry) would be ordered all industries, cars and steel. In fact the x-axis is already ordered alphabetically, but I wouldn't know how to achieve that if it were not already the case.

I feel somewhat embarrassed about having to ask this question as I know there are many similar on SO, but sorting and ordering in R remains my personal bugbear and I cannot get this to work. Although I do try, in all except the simplest cases I got lost in a welter of calls to factor, levels, sort, order and with.

Q. How can I arrange the above highlight table so that both y-axis and x-axis are ordered alphabetically?

EDIT: The answers from smillig and joran below do resolve the question with the test data but with the real data the problem remains: I can't get an alphabetical sort. This leaves me scratching my head as the basic structure of the data frame looks the same. Clearly I have omitted something, but what??

> str(mymelt)
'data.frame':   340 obs. of  3 variables:
 $ Industry: chr  "Animal and vegetable products" "Food and beverages" "Chemicals" "Plastic and rubber goods" ...
 $ variable: Factor w/ 17 levels "Other areas",..: 17 17 17 17 17 17 17 17 17 17 ...
 $ value   : num  0.000904 0.000515 0.007189 0.007721 0.000274 ...

However, applying the with statement doesn't result in levels with an alphabetical sort.

> with(mymelt,factor(variable,levels = rev(sort(unique(variable)))))

  [1] USA                   USA                   USA                  
  [4] USA                   USA                   USA                  
  [7] USA                   USA                   USA                  
 [10] USA                   USA                   USA                  
 [13] USA                   USA                   USA                  
 [16] USA                   USA                   USA                  
 [19] USA                   USA                   Canada               
 [22] Canada                Canada                Canada               
 [25] Canada                Canada                Canada               
 [28] Canada                Canada                Canada    

All the way down to:

 [334] Other areas           Other areas           Other areas          
 [337] Other areas           Other areas           Other areas          
 [340] Other areas

And if you do a levels() it seems to show the same thing:

 [1] "Other areas"           "Oceania"               "Africa"               
 [4] "Other Non-Eurozone"    "UK"                    "Other Eurozone"       
 [7] "Holland"               "Germany"               "Other Asia"           
[10] "Middle East"           "ASEAN-5"               "Singapore"            
[13] "HK/China"              "Japan"                 "South Central America"
[16] "Canada"                "USA"  

That is, the non-reversed version of the above.

The following shot shows what the plot of the real data looks like. As you can see, the x-axis is sorted and the y-axis is not. I'm perplexed. I'm missing something but can't see what it is.

Answer 1

Another possibility is to use fct_reorder from forecast library.

library(forecast)
mydf %>%
pivot_longer(cols=c('all regions', 'americas', 'europe')) %>% 
  mutate(name1=fct_reorder(name, value, .desc=FALSE)) %>% 
  ggplot( aes(x = industry, y = name1, fill = value)) +
  geom_tile() + geom_text(aes( label = value))