一、图例默认位置

默认的位置在NorthEast

r = 10;
a = 0;
b = 0;
t=0:0.1:2.1*pi;
x=a+r*cos(t);
y=b+r*sin(t);
A1=plot(x,y,'r','linewidth',4);%圆
hold on
axis equal
A2=plot([0 0],[1 10],'b','linewidth',4);%直线
legend([A1,A2],'圆形','line')

二、通过Location对legend的位置进行改变

变为North

r = 10;
a = 0;
b = 0;
t=0:0.1:2.1*pi;
x=a+r*cos(t);
y=b+r*sin(t);
A1=plot(x,y,'r','linewidth',4);%圆
hold on
axis equal
A2=plot([0 0],[1 10],'b','linewidth',4);%直线
legend([A1,A2],'圆形','line','Location','North')

三、图例放在图的其他位置

设置	位置
North	inside plot box near top
South	inside bottom
East inside right
West	inside left
NorthEast	inside top right (default for 2-D plots)
NorthWest	inside top left
SouthEast	inside bottom right
SouthWest’	inside bottom left
NorthOutside	outside plot box near top
SouthOutside	outside bottom
EastOutside	outside right
WestOutside	outside left
NorthEastOutside	outside top right (default for 3-D plots)
NorthWestOutside	outside top left
SouthEastOutside	outside bottom right
SouthWestOutside	outside bottom left
Best	least conflict with data in plot
BestOutside	least unused space outside plot

直接替换即可

四、图例放在图的外面

r = 10;
a = 0;
b = 0;
t=0:0.1:2.1*pi;
x=a+r*cos(t);
y=b+r*sin(t);
A1=plot(x,y,'r','linewidth',4);%圆
hold on
axis equal
A2=plot([0 0],[1 10],'b','linewidth',4);%直线
legend([A1,A2],'圆形','line','Location',' WestOutside')

五、只显示一个曲线

r = 10;
a = 0;
b = 0;
t=0:0.1:2.1*pi;
x=a+r*cos(t);
y=b+r*sin(t);
A1=plot(x,y,'r','linewidth',4);%圆
hold on
axis equal
A2=plot([0 0],[1 10],'b','linewidth',4);%直线
legend([A1],'圆形','Location',' WestOutside')