Sample line graph plot - what is the XY axis represent? According to the manual, the "histogram" graph plot has the X axis represents values of pixel, from 0 to 1 (or 0 to 255), while the Y axis represents the number of pixels in that tonal range. 

When I draw a "sample line" on my image, I got the graph plot that looks like it has the X axis represents the pixel point/position, while Y axis represents the brightness (luma) for each points.

There is no further explanation on the graph plot of sample line in the manual.Can someone clarify whether the XY axis for "sample line" graph plot mentioned above is correct or otherwise?
That plot you get is not an histogram. It is used for video analysis, it is called Waveform Monitor. And it is, as you correctly deduce, a plot of a spatial horizontal axis (pixel along the line), and a luminance vertical axis (brigthness for that pixel.

See the page as a reference: https://www.bhphotovideo.com/explora/video/tips-and-solutions/introduction-waveforms-scopes-and-exposure