ggplot2 - geom_segment plot in R -

i trying make plot showing different types , magnitudes of change between several types of map categories on time.

i found thread helpful can't quite want: proportionally sized arrows in ggplot

i have dataframe looks this:

class1  class2  changetype changemagnitude 1       2       green       234  1       3       green       22 2       1       red         171  2       3       green       144 3       1       red         27 3       2       red         56 

i want plot lines length of arrows or size of arrow heads proportional "changemagnitude" , want colour of arrows vary changetype (e.g., increase = green, decrease =red).

here code tried:

library(ggplot2) library(grid)  ggplot(mydata, aes(x=class1, y=class2)) + geom_segment(aes(xend=type1, yend=type2),data=mydata, colour=mydata$changetype, arrow = arrow(length =  unit(mydata$changemagnitude*1, "cm"))) 

the colour argument doesn't work @ , i'm not sure arrows giving me want. suggestions appreciated.

alternatively, may better use proportional point symbols rather arrows...