Ho più o meno risolto mettento insieme le istruzioni per i due assi.
Se ingrandisco la finestra del plot devo sistemare un pò di cose manualmente ma almeno non si incasina come prima.
il codice è
%Label assi principali - It is necessary to give the label instructions after plot in order to avoid overlap
xlabel(gca, 'Deflessione magnetica [m]'); % label lower x axis
ylabel(gca,'Deflessione elettrica [m]'); %label left y axis
%particles outside MCP radius won't appear in figure
xlim([0, Rmcp])
ylim([0, Rmcp])
%%%% legenda assi principali
l=legend(hplot, 'H^+','C^+','C^{+2}','C^{+3}','C^{+4}','C^{+5}','C^{+6}', 'Location','BestOutside');
a=get(l,'children');
set(a(1:3:end),'MarkerSize',10);
%%%% doppi Assi
%set secondary x limit as the momentum of a H+ at distance equal to the MCP radius
mv_max = (q*B*LB*L)/Rmcp;
%set secondary y limit as the energy of a H+ at distance equal to the MCP radius
Le = 0.07; %Estensione del C.E. : 70 mm
E = 100000; %campo TP.m
Ekin_max = (q*E*Le*L)/(2*m_H*Rmcp);
%Layout instruction
set(gca,'Box','off'); % Turn off the box surrounding the whole axes
%axesUnits=get(gca,'Units');
axesPosition = get(gca,'Position'); %# Get the current axes position
hNewAxes = axes('Position',axesPosition,... %# Place a new axes on top...
'Units', 'normalized',...
'ActivePositionProperty', 'Position',... %prova anche OuterPosition - ma sembra peggio
'Color','none',... %# ... with no background color
'YAxisLocation', 'right',...
'XAxisLocation','top',... %# ... located on the top
'Ylim', [0, Ekin_max],...
'Xlim', [0, mv_max],... %# ... should define x axis scale (need to set xmax = mv_max)
'Box','off'); %# ... and no surrounding box
xlabel(hNewAxes,'Momentum (H^+)'); %# Add a label to the top axis
ylabel(hNewAxes,'Energy (H^+)'); %# Add a label to the right axis
set(gca, 'XTickLabel', num2str(get(gca,'XTick')','%g'))
set(gca, 'YTickLabel', num2str(get(gca,'YTick')','%g'))
%%%%%Plot title - It is necessary to give the title instruction after secondary x axis in order to avoid overlap
title(['Calcolo approssimato interazione ioni campo magnetico B=', num2str(B), 'Tesla']);