以不同颜色打印x轴上方的y值

我在这里策划 ((2*x.^2+3*exp(-x) -(100*a)).*sin(5*x))./(6*x.^2-9*x-42)-10 输入 a=3 , dx=0.1 .

我想用蓝色绘制所有正y值,用红色绘制所有负y值:

a = input('Please input a: ');
dx = input('Input step size dx: ');
if dx<0
   fprintf('dx must be a non-negative number.');
   while(dx<0)
       dx = input('Input step size dx: ');
   end
end
clf;
x = -10:dx:10;
y = ((2*x.^2+3*exp(-x) -(100*a)).*sin(5*x))./(6*x.^2-9*x-42)-10

plot(x,y)
ylim([-100,100])

为了用蓝色绘制正y值,用红色绘制负y值,我尝试了初始化正y值及其域的向量,初始化负y值及其域的向量。

s = size(y);
x_1 = [];
x_2 = []; %negative
y_1 = [];
y_2 = []; %negative
for i = 1:s(2)
    if(y(i) >0)
        x_1 = [x_1,x(i)];
        y_1 = [y_1,y(i)];
    elseif (y(i) <0)
        x_2 = [x_2,x(i)];
        y_2 = [y_2,y(i)];
    end
end

s_x1 = size(x_1);
s_x1_lim = s_x1(2);
s_x2 = size(x_2);
s_x2_lim = s_x2(2);

plot(x_1,y_1,'b');
xlim([x_1(1), s_x1_lim]);
hold on
plot(x_2,y_2,'r');
xlim([x_2(1), s_x2_lim]);
hold on;
xlim([-10,10])
ylim([-100,100]);

问题是,这种方法会留下一些我不希望有的重叠。我怎样才能改变这个?