Highlight a specific point in the graph

[Leandro Sales]

Please, suppose the following example:

I have a graph x=time, y=throughput. Consider the workload below,
where in the first column we have the time in seconds and in the
second column the throughput in Kbits/s (just to contextualize):

x  y
-------
1  10
2  12
3  15
4  1
5  16
6  13
7  14
8  17
9  15

In this example realize that in x=4 seconds the throughput is so
allow, y=1 Kbits/s, if we compare with the other (x, y) points, where
the throughput ranges between 12 and 17 Kbits/s. I want to plot a
graph and highlight (using a arrow, a triangle, a circle, ...) that
point (4, 1). Is this possible? I expected that octave give me a way
to flag that points, something like: "highlight that point, that point
and that point", something like:

1  10
2  12
3  15
4  1   HIGHLIGHT
5  16
6  13
7  14
8  17 HIGHLIGHT
9  15

... and when octave finds (4, 1) and (8,17) it highlights that points.

Thank you in advance. Any comment/clue will be greatfully appreciated.

Cheers,
Leandro.