Try this:
x = (1:10)';
y = [2.14 3.66 4.55 4.55 4.55 4.55 8.89 8.89 14.43 14.43]';
plot(x, y, 'b.-', 'MarkerSize', 30);
grid on;
xticks(1:10);
% Predict the value at x=6.5
xi = 6.5;
yi = interp1(x, y, xi)
% Plot interpolated point in green.
hold on;
plot(xi, yi, 'g.', 'MarkerSize', 40);
% Draw lines.
line([xi, xi], [0, yi], 'Color', 'g', 'LineWidth', 2);
line([0, xi], [yi, yi], 'Color', 'g', 'LineWidth', 2);
