Getting incorrect plot – curve does not cross the x axis correctly

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Hello,

I'm trying to plot a quadratic equation with highlighted roots and I am facing a problem – my curve does not cross x axis correctly.

For example:

y = 2x^2 - 4x, => roots are x1 = 2, x2 = 0 and I get this graph:

So the blue curve should cross the red points but does not. I dont know why.

My code for plotting this is like this:

x = [min(x1, x2)-5:0.1:max(x1, x2)+5];
y = (koef_a*x).^2 + koef_b*x + koef_c;
plot(x, y, 'b'), grid on, xlabel('x'), ylabel('y');
ax = gca; % Get handles to axis.
ax.YAxisLocation = 'origin';
ax.XAxisLocation = 'origin';
hold on;
plot(x1, 0, 'o', 'MarkerSize', 5, 'MarkerFaceColor', 'r', 'MarkerEdgeColor', 'r');
plot(x2, 0, 'o', 'MarkerSize', 5, 'MarkerFaceColor', 'r', 'MarkerEdgeColor', 'r');
hold off;