[R] How to denote a line on the graph

[aegea]

Please give me some help, many thanks.

I graphed a expected CDF line of a binomial distribution on a graph,
And I have some observed points (observed CDF) from 4 groups fall on the
smooth CDF line.

I cannot really get the legend I want


legend ('topleft', c('a, 'b', 'c', 'd', 'expected CDF'), col=c(1,2,3,4),
pch=c(0,1,2, 3, '' ), lty=c(0,0,0,0,1) )
 
Ideally, in legend I want to show different graphics marks for different
group, and a short line in the legend to show the smooth line on the graph
is the CDF. but, the legend with my code shows 0, 1, 2, 3 instead different
graph notations for the four groups.  
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