Line Rendering Problem

I have a set of numbers that, when plotted, causes the linestyle to be displayed incorrectly.

The code below gives my dataset and plots them in subplot 1, the second subplot is an identical line (same max,min and number of datapoints in x and y), but with the datapoints evenly spaced along the line.

On my system (Matlab 7.11.0.584 (R2010b) running on Mac OS X 10.7.1) the line in the first plot is not displayed correctly, while it is in the second. Mysterious!

Any ideas?

Here's a picture of what I get... https://files.me.com/oscarbranson/siin06

% My data set
subplot(2,1,1)
x1=[4.98270000000000,4.95770000000000,4.94240000000000,4.95240000000000,4.93420000000000,4.98760000000000,4.98550000000000,4.95800000000000,4.96170000000000,4.94190000000000,4.98210000000000,4.98610000000000,4.95470000000000,4.96070000000000,4.93870000000000,4.98770000000000,4.98130000000000,4.98130000000000,4.98140000000000,4.98140000000000,4.98160000000000,4.98160000000000,4.98130000000000,4.98130000000000,4.98150000000000,4.98150000000000,4.98080000000000;];
y1=[17.0600972366516,16.9321690873624,16.8538770599974,16.9050483197131,16.8119166270306,17.0851711539123,17.0744251893720,16.9337042251539,16.9526375912487,16.8513184970117,17.0570269610687,17.0774954649550,16.9168177094477,16.9475204652771,16.8349436939026,17.0856828665095,17.0529332602914,17.0529332602914,17.0534449728886,17.0534449728886,17.0544683980829,17.0544683980829,17.0529332602914,17.0529332602914,17.0539566854857,17.0539566854857,17.0503746973056;];
plot(x1,y1,'r--')

% A generated line with the same number of data points and the same x and y
% limits.
subplot(2,1,2)
x2=min(x1):((max(x1)-min(x1))/(length(x1)-1)):max(x1);
y2=min(y1):((max(y1)-min(y1))/(length(y1)-1)):max(y1);
plot(x2,y2,'r--')

[Fangjun Jiang]

What is your code?

Answer 1

[Oleg Komarov]

Your x1 are not sorted:

[x1,idx] = sort(x1);
plot(x1,y1(idx),'r--')

What happens is that the coordinates trace a trajectory that goes back and forth and the dashed sequence of the run in one direction can be overwritten by the a change in direction.

[Fangjun Jiang]

@Oleg, I think your explanation is more to the point (+1). My explanation also provides some insight. The two curves are different. They should not be expected to be the same. In this case they look same only because the line happens to be almost straight.

[Oscar Branson]

Thanks! So simple...

Answer 2

[Fangjun Jiang]

I think it's expected. If you change your plot command (both) to be plot(x1,y1,'rs'), you will see the effect. (x1,y1) is not evenly spaced. The line between is the interpretation. If you change it to be plot(x1,y1,'r-'), they look more or less the same.