gnuplot-info

[Gnuplot-info] request for help plotting surface (pm3d) with data file -- no data are being plotted

[D. R. E. <doc...@gm...>]

Using gnuplot 4.0, patch level 0.

I apologise for this elementary question, but I have spent more than
an hour trying to do something very simple, with no success (and also
spent time additional googling for an example, to no avail).

I have a data file in a simple x y z format. It starts like this:

1 0 0
1 10 0
1 20 0
1 30 0
1 40 0
1 50 0
1 60 0
1 70 0
1 80 0

and ends like this

249 24950 0
249 24960 0
249 24970 1
249 24980 0
249 24990 1

It contains a total of ~350,000 lines.

If I start gnuplot and simply run "splot <filename>" I get a plot, so
the file is being read properly.

This simple plot, though, cannot really be used, since it's just a
mass of red points with no way to see surfaces or anything useful.

So I figured that the "pm3d" capability is probably what I need.

But if I type:
set pm3d
splot <filename>  with pm3d
I get a set of orthogonal axes, a pretty colour bar, and no points or
surfaces plotted.

I don't understand why I'm not seeing anything.  So I have two simple questions:

1. Why don't I see any points/surface plotted?
2. What commands do I need to issue in order to see the data?

  Doc Evans

Re: [Gnuplot-info] request for help plotting surface (pm3d) with data file -- no data are being plotted

[D. R. E. <doc...@gm...>]

D. R. Evans said the following at 11/11/2006 01:17 PM :

> 1. Why don't I see any points/surface plotted?
> 2. What commands do I need to issue in order to see the data?
>

I finally figured out that one needs to add a blank line whenever the value
in the first column changes. I don't understand why this has to be done
manually, but apparently it does.

  Doc

Re: [Gnuplot-info] request for help plotting surface (pm3d) with data file -- no data are being plotted

[Lars H. <la...@ho...>]

On Saturday November 11th 2006 21:17 D. R. Evans wrote:
> I have a data file in a simple x y z format. It starts
> like this:
>
> 1 0 0
> 1 10 0
> 1 20 0
> 1 30 0
> 1 40 0
> 1 50 0
> 1 60 0
> 1 70 0
> 1 80 0
>
> and ends like this
>
> 249 24950 0
> 249 24960 0
> 249 24970 1
> 249 24980 0
> 249 24990 1
>
> It contains a total of ~350,000 lines.

Does your file only contain the z-values 0 and 1? As you've 
described, pm3d does not produce the output you hoped it 
would be. As I see it, the problem is in the structure of 
your dataset. 

Most of the z-values are 0, that should produce a flat 
surface with the darkest color of the color bar. Over this 
surface there are only 'a few' peaks with the value 1. The 
pm3d-option tries to connect these peaks with the 0-surface 
on the bottom. Due to the mathematic constellation of your 
data, the resulting connecting surfaces have to be like  
infinte small columns standing rectangular on the base 
surface. Since there are no values given in between, 
gnuplot has problems, how to color these (no-width) 
columns.

As a consequence only small dots with the brightest colors 
of the colormap should be drawn at the spots, where your 
dataset contains a 1. Since you are plotting 350.000 
datapoints, these dots may te too short for the reolution 
of your screen (or your favoured output medium). 

My advice: Reduce the number of datapoints to a plottable 
amount; you may use gnuplots facilities to plot only every 
nth line of your dataset (e.g splot dataset every 3). This 
in combination with a change of the plotting symbol (e.g. 
splot dataset with dots) may give a better result.

Greetings
Lars