Live updates are a bit more tricky, but you can use the uicontrol function to create a slider. Then you can use the callback function to change the YData according to your selected voltage. (changing the YData property of you line object is much faster than clearing the axes and calling plot again).
Let me know if you need a small example.
Edit:
The example below shows how you could create a GUI that helps you find out the effect of changing a particular value in polyval. This examples works with continuous data, but your situation might not. You can use round on the Value property to ensure interger outputs.
h=struct;%prepare guidata struct
h.f=figure(999);clf(h.f)%open a clean figure
h.ax=axes('Parent',h.f,...
'Units','Normalized',...
'Position',[0.1 0.2 0.8 0.75]);
%prepare data
h.x=linspace(0,1,1000);
h.p=[6.5 -15 NaN -3 0];
%initialize plot
p=h.p;p(3)=0;
y=polyval(p,h.x);
h.line=plot(h.x,y,'Parent',h.ax);
%create slider
h.slider=uicontrol('Parent',h.f,...
'Units','Normalized',...
'Position',[0.1 0.05 0.8 0.09],...
'Style','slider',...
'Min',5,'Max',15,'Value',10,...
'Callback',@slider_callback);
h.legend=legend(h.line,sprintf('V=%.1f',p(3)));
%store handles and data to guidata
guidata(h.f,h)
function slider_callback(hObject,eventdata) %#ok<INUSD>
h=guidata(hObject);%retrieve struct
p=h.p;p(3)=get(hObject,'Value');%set slider value to p(3)
y=polyval(p,h.x);
set(h.line,'YData',y)%update plot
set(h.legend,'String',{sprintf('V=%.1f',p(3))})%update legend
end
