There are a few ways to do this. With annotation, it's easy to combine an arrow with a text box. The downside is you have to use normalized coordinates, rather than data coordinates:
ax = gca;
xValue = 5;
axis([0, 10, 0, 10]);
xline(xValue);
x2 = xValue / ax.XLim(2) * ax.Position(3) + ax.Position(1);
x1 = x2 - 0.1; % on left side of line
y2 = .5; % middle of line in y-axis
y1 = .5; % middle of line in y-axis
annotation('textarrow', [x1 x2], [y1 y2], 'String', sprintf('x = %d ', xValue));
