hold on 使當前軸及圖形保持而不被刷新，准備接受此后將繪制的圖形，多圖共存
hold off 使當前軸及圖形不在具備被刷新的性質，新圖出現時，取消原圖
hold on 在當前圖的軸（坐標系）中畫了一幅圖，再畫另一幅圖時，原來的圖還在，與新圖共存，都看得到
close all
x=0:2:100;
y1=2*x;
y2=3*x;
y3=3.5*x;
ph1=plot(x,y1,'r-'); hold on
ph2=plot(x,y2,'g--');
ph3=plot(x,y3,'b:');
title('fake data')
xlabel('my x data');
ylabel('my y data');
th1=text(x(13),y1(13),'red line');
th2=text(x(15),y2(15),'green line');
th3=text(x(17),y3(17),'blue line');
set(th1,'Color','r');
set(th2,'Color','g');
set(th3,'Color','b')
lh=legend([ph1 ph2 ph3],{'x2';'x3';'x3.5'})
x=0:2:100;
y1=2*x;
y2=3*x;
y3=3.5*x;
ph1=plot(x,y1,'r-'); hold on
ph2=plot(x,y2,'g--');
ph3=plot(x,y3,'b:');
title('fake data')
xlabel('my x data');
ylabel('my y data');
th1=text(x(13),y1(13),'red line');
th2=text(x(15),y2(15),'green line');
th3=text(x(17),y3(17),'blue line');
set(th1,'Color','r');
set(th2,'Color','g');
set(th3,'Color','b')
lh=legend([ph1 ph2 ph3],{'x2';'x3';'x3.5'})
hold off 在當前圖的軸（坐標系）中畫了一幅圖，此時，狀態是hold off,則再畫另一幅圖時，原來的圖就看不到了，在軸上繪制的是新圖，原圖被替換了
close all
x=0:2:100;
y1=2*x;
y2=3*x;
y3=3.5*x;
ph1=plot(x,y1,'r-'); hold off
ph2=plot(x,y2,'g--');
ph3=plot(x,y3,'b:');
title('fake data')
xlabel('my x data');
ylabel('my y data');
th1=text(x(13),y1(13),'red line');
th2=text(x(15),y2(15),'green line');
th3=text(x(17),y3(17),'blue line');
set(th1,'Color','r');
set(th2,'Color','g');
set(th3,'Color','b')
lh=legend([ph1 ph2 ph3],{'x2';'x3';'x3.5'})
x=0:2:100;
y1=2*x;
y2=3*x;
y3=3.5*x;
ph1=plot(x,y1,'r-'); hold off
ph2=plot(x,y2,'g--');
ph3=plot(x,y3,'b:');
title('fake data')
xlabel('my x data');
ylabel('my y data');
th1=text(x(13),y1(13),'red line');
th2=text(x(15),y2(15),'green line');
th3=text(x(17),y3(17),'blue line');
set(th1,'Color','r');
set(th2,'Color','g');
set(th3,'Color','b')
lh=legend([ph1 ph2 ph3],{'x2';'x3';'x3.5'})
