Is there a list of lx objects I am able to extract with the --objects option?

To get the list of supported objects run lx with no --objects option. You get a lengthy output that contains all available objects. E.g. FTP-related objects:

  ftp.rptm.count: 0
  ftp.active.rptm.sum: 0.00
  ...
  ftp.size.count: 0
  ...
  ftp.active.rptm.count: 0
  ...

Now you can limit the lx output to FTP-related objects only using "--objects ftp". Or further limit it to active FTP transactions with "--objects ftp.active" or to active FTP transactions size stats with "--objects ftp.active.size". And so on.

Unfortunately, there is no up-to-date documentation available for what each object is. There is outdated documentation about lx output [1] and options [2]. But it is of limited usage now.

The most common stats are rptm and size. Rptm is response time stats and size is message size stats. Most important values are:

.count - number of messages
.min/.max/.mean - min/max/mean response time or message size
sum - sum of all response times or message sizes

Many rptm and size stats also have a histogram.

Polygraph collects stats for many different kind of transactions. The 'rep.' category is the most general one and collects stats for all replies. So 'rep.rptm.mean' is the mean response time for all transactions. There are many more transaction categories, e.g. basic.* (basic transactions), head.* (HTTP HEAD transactions), ssl.* (HTTPS transactions).

Note that lx output is intended mostly for automated processing and quick checks by humans. Consider using the reporter tool for generating a human-friendly test report.

[1] http://polygraph.ircache.net/doc/lxoutput.html
[2] http://polygraph.ircache.net/doc/lxopts.html