You don't explain what you don't like nor show us what you got and we don't have your data, so we're flying blind here, but I'll guess
plot([-x1(:);x1(:)],[y1(:);y1(:)],'o-r');
hold on;
plot(x2,y2);
legend('plotx1y1','plotx2y2');
or
hL=gobjects(3,1); % preallocate for line handles to come
hL=plot(x1,y1,'o-r',-x1,y1,'o-r'); % first two
hold on;
hL(3)=plot(x2,y2); % and third...
legend(hL([1 3]),'plotx1y1','plotx2y2'); % label the desired ones
or
plot(x1,y1,'o-r','DisplayName','plotx1y1','Annotation','on'); % first, use legend
hold on;
plot(-x1,y1,'o-r','Annotation','off'); % no legend on second
plot(x2,y2,'DisplayName','plotx2y2','Annotation','on'); % show third
legend % turn the legend on
