You could do the following:
xtip=0.5; ytip=0.5; % arrow tip coordinates (normalized units)
w=0.15; %box width
h=0.1; %box height
offset=0.1;
str=['0.8042 p.u.' char(10) '15:05:20.133'];
x = [xtip-offset xtip]; % arrows start and end coordinates
y = [ytip+offset ytip]; % I've just offset by 0.1 in x and y.
a=annotation('textarrow',x,y,'Color','red');
b=annotation('textbox',[xtip-w-0.1 ytip+0.1 w h],'String',str,'Color','red','EdgeColor','red');
The coordinates here are normalised with respect to your axes limits. For directly specifying coordinates w.r.t the data you used in the plot, like a point on your line, I would look at the following question.
