How to manually add a legend to a ggplot object

Question

I have this data frame:

structure(list(month_num = 1:24, founded_month = c(4L, 5L, 6L, 
7L, 8L, 9L, 10L, 11L, 12L, 1L, 2L, 3L, 4L, 5L, 6L, 7L, 8L, 9L, 
10L, 11L, 12L, 1L, 2L, 3L), founded_year = c(2008L, 2008L, 2008L, 
2008L, 2008L, 2008L, 2008L, 2008L, 2008L, 2009L, 2009L, 2009L, 
2009L, 2009L, 2009L, 2009L, 2009L, 2009L, 2009L, 2009L, 2009L, 
2010L, 2010L, 2010L), count = c(270L, 222L, 256L, 250L, 277L, 
268L, 246L, 214L, 167L, 408L, 201L, 225L, 203L, 220L, 230L, 225L, 
177L, 207L, 166L, 135L, 116L, 122L, 69L, 42L), month_abb = c("Apr", 
"May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec", "Jan", 
"Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", 
"Nov", "Dec", "Jan", "Feb", "Mar"), short_year = c("08", "08", 
"08", "08", "08", "08", "08", "08", "08", "09", "09", "09", "09", 
"09", "09", "09", "09", "09", "09", "09", "09", "10", "10", "10"
), proj = c(282, 246, 292, 298, 337, 340, 330, 310, 275, 528, 
333, 369, 359, 388, 410, 417, 381, 423, 394, 375, 368, 386, 345, 
330), label = c("Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", 
"Nov", "Dec", "Jan\n09", "Feb", "Mar", "Apr", "May", "Jun", "Jul", 
"Aug", "Sep", "Oct", "Nov", "Dec", "Jan\n10", "Feb", "Mar")), .Names = c("month_num", 
"founded_month", "founded_year", "count", "month_abb", "short_year", 
"proj", "label"), row.names = c(NA, -24L), class = "data.frame")

and i've got all of this done (I know the code's a bit ugly looking, pointers appreciated):

p <- ggplot(m_summary2, aes(x = month_num, y = count))
p + 
geom_line(colour = rgb(0/255, 172/255, 0/255)) + geom_point(colour = rgb(0/255, 172/255,          
    0/255)) + 
geom_line(aes(x = m_summary2$month_num, y = m_summary2$proj), 
    colour = rgb(18/255, 111/255, 150/255)) + 
geom_point(aes(x = m_summary2$month_num, y = m_summary2$proj), colour = rgb(18/255,   
    111/255, 150/255)) +     
scale_x_continuous("Month", breaks = m_summary2$month_num, labels = m_summary2$label) + 
scale_y_continuous("# Startups Founded") + 
opts(title = paste("# Startups Founded:", m_summary2$month_abb[1], 
    m_summary2$short_year[1], "-", m_summary2$month_abb[nrow(m_summary2)],  
    m_summary2$short_year[nrow(m_summary2)]))

Now I would like to add a legend to clarify that the blue line is a projection and the green line is the current data. I would like to make the changes without altering the dataframe if possible.

Thanks in advance!

Answer 1

You can easily achieve this by using melt (in the reshape package). Here is the code you add after you define the data frame.

id1 = c("month_num","founded_month", "founded_year","month_abb","short_year","label");   
m_summary3 = melt(m_summary2, id = id1);
p = ggplot(m_summary3, aes(x = month_num, y = value, group = variable, colour = variable));
c1 = rgb(0/255, 172/255, 0/255);
c2 = rgb(18/255, 111/255, 150/255);
x_scale = scale_x_continuous("Month", breaks = m_summary2$month_num, labels = m_summary2$label);
y_scale = scale_y_continuous("# Startups Founded")

p + geom_line() + scale_colour_manual(values = c(c1,c2)) + x_scale + y_scale;

Ramnath