Symbols work in plot but line doesn't

When I choose to display the data in the graph as a symbol (e.g. "*" or "x" or "o") it comes out fine, but when I try to get it to display as a line my graph is blank. My Redlich_Kwong function is working and returning v successfully. Any help as to how to connect up my data points to get a line?

Pressure_vector = [1 1.5 2 2.5 3 5 10 15 25 50 100];

%%%%DATA IS PLOTTED%%%%
for i = 1:11
    P = Pressure_vector(1,i);
    [v,err,counter] = Redlich_Kwong(R,T,Tc,P,Pc);
    plot(P,v,'b*');
    hold on;
end

%%%%NO DATA IS PLOTTED%%%%
for i = 1:11
    P = Pressure_vector(1,i);
    [v,err,counter] = Redlich_Kwong(R,T,Tc,P,Pc);
    plot(P,v,'b-');
    hold on;
end

Answer 1

[Adam Danz]

I assume P and v are both scalar values. If that's the case, the definition of a line requires at least two value. For example, you can draw the point (3,1) but it doesn't make sense to draw the line at (3,1). It would make sense to draw a line from (3,1) to (4,2).

A better approach would be to store the values in the loop and then plot them later. Then you can plot a line.

P = nan(1,11); 
v = nan(1,11); 
for i = 1:11
    P(i) = Pressure_vector(1,i);
    [v(i),err,counter] = Redlich_Kwong(R,T,Tc,P(i),Pc);  % assuming output is scalar
end

plot(P,v,'b-');