Best-Fit line only extends in the positive x-axis direction

Alright so I've written this piece of code in Matlab with the goal of having a line of best fit plotted in place of my two data arrays so that I can see where the line crosses the x&y axes visually. The only problem is, the line of best fit only extends in the positive x-axis direction, so it never actually crosses the x and y axes. What am I missing?

y = [200 300 400 500];      % Lambda
invy = 1./y;                %1/Lambda
V = [4.00 1.86 0.85 0.21];  % Vo
q = 1.6e-19;                % Charge of Electron

p = polyfit(invy,V,1);
f = polyval(p,invy);

figure(1)
plot(invy,f)
xlim([-5e-3 5e-3])
ylim([-2 2])
grid on

Answer 1

[Image Analyst]

Try this:

y = [200 300 400 500];      % Lambda
invy = 1./y;                %1/Lambda
V = [4.00 1.86 0.85 0.21];  % Vo
q = 1.6e-19;                % Charge of Electron
hFig = figure;
plot(invy, V, 'b.', 'MarkerSize', 50);
grid on;
hFig.WindowState = 'maximized';
% Determine the coefficients of the line.
coefficients = polyfit(invy, V, 1)
fprintf('Equation of the line is: V = %f * invy + %f.\n', coefficients(1), coefficients(2));
% Now do a fit from -xmax to +xmax, or whatever you want.
xmax = abs(max(invy));
xFit = linspace(-xmax, xmax, 800); % 800 points, or however many you want.
yFit = polyval(coefficients, xFit);
hold on;
plot(xFit, yFit, 'b-', 'LineWidth', 2);
xlim([-xmax, xmax])
xlabel('invy', 'FontSize', 20);
ylabel('V', 'FontSize', 20);
% ylim([-2 2])
grid on;
% Put lines along axes.
xline(0, 'LineWidth', 2, 'Color', 'k');
yline(0, 'LineWidth', 2, 'Color', 'k');
% If you want tick marks inside rather than outside the graph, do this:
ax = gca;
ax.XAxisLocation = 'origin';
ax.YAxisLocation = 'origin';

[Bryant Schisel]

Thank You!!