Q:

How can I adjust the axes to start from zero origin in r plot

To plot the empirical cumulative density of three variables, x1, x2 and x3, I used the following in r: 
plot.ecdf(x1, col="blue",
     main="Distribution XYZ",
     xlab="x_i", ylab="Prob(x_i<=y)")

lines(ecdf(x2), col="red")    # adds a line
lines(ecdf(x3), col="green")  # adds line
legend(600,0.6, # places a legend from (x,y)=(600,0.6) on
       c("x1","x2","x3"), # puts text in the legend
       lty=c(1,1,1), # gives the legend appropriate symbols (lines)
       lwd=c(1,1,1),col=c("blue","red","green")) # gives the legend lines the correct color and width

The resulting plot however has two horizontal lines (broken lines) at 0 and 1 besides the box. And, the origin of the box has a space below zero on the vertical axis and a space to the left of zero on the horizontal axis. May you suggest how to remove these space and the additional lines. I wanted but could not post the plot. 
EDITED:
A sample data can be generated as follows:
sample data
n <- 1000; u <- runif(n)
a <- -4.46; b <- 1.6; c <- -4.63
d <- ( a * u ) + (b * ( ( 1.5 * ( u ** 2 )) - 0.5 ))  + (c * ( (2.5 * (u ** 3)) - (1.5 * u )))
x1 <- -126/d; x2 <- -131/d; x3 <- -187/d

A:

It sounds like you are asking for the style provided by different settings for 'xaxs' and 'yaxs', and maybe 'xlim':
Try:
 plot.ecdf(x1, col="blue",
     main="Distribution XYZ",
     xlab="x_i", ylab="Prob(x_i<=y)", 
     xaxs="i",yaxs="i", xlim=c(0,1000) )

lines(ecdf(x2), col="red")    
lines(ecdf(x3), col="green")  
legend(600,0.6, 
       c("x1","x2","x3"), 
       lty=c(1,1,1), 
       lwd=c(1,1,1),col=c("blue","red","green"))

A:

use the yaxs = "i" argument:
plot.ecdf(x1, col="blue",
     main="Distribution XYZ",
     xlab="x_i", ylab="Prob(x_i<=y)", yaxs = "i")

This will align the axis at the ylim points (which supposedly are 0 and 1 for plot.ecdf).
If you also want to remove the dotted line at 0 and 1, just call box():
box()

