I would like create a Matlab figure with a double X axis (m/s and km/h) with the same plot.

I have found plotyy and - in Matlab reposity - plotyyy, but I am looking for:

1. A double X axis.
2. Together below the plot.

My code is very simple: 

 stem(M(:, 1) .* 3.6, M(:, 3));

grid on

xlabel('Speed (km/h)');
ylabel('Samples');

M(:, 1) is the speed (in m/s), and M(:, 3) is the data.

I would like only a second line, in the bottom, with the speeds in m/s. 

You can do something like the following. In comparison to the solution of @Benoit_11 I do use the normal Matlab labels and refer to both axes with handles so the assignments are explicit. 

The following code creates an empty x-axis b with the units m/s with a negligible height. After this, the actual plot is drawn in a second axes a located a bit above the other axes and with units km/h. To plot on a specific axes, insert the axes-handle as the first argument of stem. The conversion from m/s to km/h is directly written in the call to stem. Finally, it's needed to set the xlim-property of the both axes to the same values. 

% experimental data
M(:,1) = [ 0,  1,  2,  3,  4,  5];
M(:,3) = [12, 10, 15, 12, 11, 13];

% get bounds
xmaxa = max(M(:,1))*3.6;    % km/h
xmaxb = max(M(:,1));        % m/s


figure;

% axis for m/s
b=axes('Position',[.1 .1 .8 1e-12]);
set(b,'Units','normalized');
set(b,'Color','none');

% axis for km/h with stem-plot
a=axes('Position',[.1 .2 .8 .7]);
set(a,'Units','normalized');
stem(a,M(:,1).*3.6, M(:,3));

% set limits and labels
set(a,'xlim',[0 xmaxa]);
set(b,'xlim',[0 xmaxb]);
xlabel(a,'Speed (km/h)')
xlabel(b,'Speed (m/s)')
ylabel(a,'Samples');
title(a,'Double x-axis plot');