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# How to create a graph

Asked by John on 1 Oct 2012

Hi,

Could somebody advise me on how you would produce a graph like this in matlab

https://dl.dropbox.com/u/54057365/All/effmap.JPG

Thank you

## 1 Comment

Azzi Abdelmalek on 1 Oct 2012

plot a graph from what?

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Answer by per isakson on 1 Oct 2012
Edited by per isakson on 31 Oct 2012

My approach, which requires some work,

• put data in a 2D array, C
• plot with the function, image
• Image type: Indexed (colormap)
• Define a colormap, with a special slot, e.g. 1, for "missing data", white in your case. The length of the colormap = 64 is enough.
• Map C carefully to the colormap, e.g. C(ii,jj)=1 for "missing data"
• User defined tick-labels on the axes
• and more

--- in response to comments I try to expand a bit ---

Variables

• cmap is a colormap of size [64x3]. It associates the numbers [1,2,..., 64] to 64 different colors. (64 is an example.)
• D is your 2D data array
• C is an array of the same size as D. The values of C are integers in the interval, [1,64].

You create a suitable colormap, cmap, with the colormapeditor. You add some specific colors to signal special values of D, e.g. "missing data", outliers of various kinds etc.

You define a function, which takes D as input and returns C, e.g.

`    function C = D2C( D )`
`        C = zeros( size(D) ); % zeros will cause error if not replaced`
```        C( isnan(D) )   = 1; % nans will e displayed with color #1
C( D >= upper ) = 2; % values above upper gets color #2
C( D <= lower ) = 3; % values below lower gets color #3
etc.
any command is ok as long as the values of C are integers [1,64]```
```        assert( not( any( C(:)==0 ) ), .... )
end ```

and to display the graph

```    colormap( cmap )
image( C )```

.

--- in response to John's comment on 19 Oct 2012 at 16:45 ---

Here is a start to make a graph that resembles https://dl.dropbox.com/u/54057365/All/eff.JPG

```    %% Create some data
N = 1000;
Speed       = transpose( logspace( 0, 2, N ) );
Acc         = randn( N, 1 ) .* ( exp( - sqrt( 10*Speed/N ) ) );
Efficiency  = ( Acc .* Speed );```
```    %%
nAccBins    = 20;   % Set resolution of image
nSpeedBins  = 30;```
```    Speed_upper = 100;
Speed_lower = 0;
Acc_upper   = 3;
Acc_lower   = -3;```
```    ix_Speed    = ceil( nSpeedBins * ( Speed - Speed_lower )        ...
/ ( Speed_upper - Speed_lower ) );```
```    ix_Acc      = ceil( nAccBins * ( Acc - Acc_lower )       ...
/ ( Acc_upper - Acc_lower ) );```
```    %%
M   = accumarray( { ix_Speed, ix_Acc }, Efficiency, [], @mean, nan );```
```    %%
imh = imagesc( transpose( M ) );
axh = ancestor ( imh, 'Axes'  );
set( axh, 'YDir', 'normal' )```

Next step would be to map M to a special colormap with something like

`    C = D2C( M );`

and display with

```    colormap( cmap )
image( C )```

.

Backlog:

• axis, ticks and ticklabels
• colorbar

per isakson on 15 Oct 2012

See above

John on 19 Oct 2012

Hello Per Isakson,

Thank you for your help with this. Would you mind if I asked you one further question please?

If I was trying to create a graph like this:

https://dl.dropbox.com/u/54057365/All/eff.JPG

So I have 3 sets of data, acceleration, speed and efficiency.

For example Speed =40 and acceleration = 2 has an efficiency of 80%.

Would acceleration and speed be in the 2D array on their own? or would this become a 3D array with acceleration, speed and efficiency?

Thank you

John

per isakson on 31 Oct 2012

See above

Answer by Image Analyst on 1 Oct 2012

You would create an image - basically a 2D array. You must have that. If you don't have that, then you have nothing at all to display. Then display it with image() or imshow(), and apply a colormap. Not hard at all, assuming you have the data, temperature vs. current, in a 2D array already.

## 1 Comment

John on 15 Oct 2012

Hello,

Thanks for your reply. I'm sorry but I don't understand how to get the colours to appear correctly.

This is what I am getting

https://dl.dropbox.com/u/54057365/All/effmat.JPG

but this is what I'm trying to achieve

https://dl.dropbox.com/u/54057365/All/efficiency.JPG

Also how do you switch the axis?