Not getting proper upstream traffic

Yes check the maxbytes in MRTG, just crank it up to something larger. 125000 = 1Mbps, so as rmadmin said above, set it to something large enough else MRTG will perhaps chop off your graphs.

Here are some examples of what bandwidth is available with the various widths and QAMs :

Example settings for DOCSIS 1.x :

1.6Mhz at QPSK = 2.56M less overheads = 2.2M usable
1.6Mhz at 16QAM = 5.12M less overheads = 4.4M usable
3.2Mhz at QPSK = 5.12M less overheads = 4.4M usable
3.2Mhz at 16QAM = 10.24M less overheads = 8.9M usable

Example settings for DOCSIS 2.0 :

3.2Mhz at 64QAM = 15.36M less overheads = 13.3M usable
6.4MHz at 16QAM = 20.48M less overheads = 17.8M usable
6.4Mhz at 32QAM = 25.60M less overheads = 22.2M usable
6.4Mhz at 64QAM = 30.72M less overheads = 26.7M usable

If you are testing with a single modem only, you may have trouble achieving high speeds unless you have your CMTS and the CM config files setup 100% correctly. Often the per-modem ceiling is about 2 to 3Mbps in that case. However if you have heaps of modems on the upstream you should certainly see your graphs go up to the speeds quoted above.

I have posted some examples previously showing the important knobs to tweak to get high per-user speeds. I'll see if I can dig up a link for you.

To give you a rough idea of what per-modem speeds are possible here's a little bit of info to get you started :

3.2width 16qam = aggregate 8.9Mbps available
with default minislot-size 2 = peak modem speed 5.6Mbps
we would tweak minislot-size to 4 = peak modem speed 7.6Mbps

3.2width 64qam = aggregate 13.3Mbps available
with default minislot-size 2 = peak modem speed 8.5Mbps
we would tweak minislot-size to 4 = peak modem speed 11Mbps

6.4width 16qam = aggregate 17.8Mbps available
with default minislot-size 1 = peak modem speed 5.7Mbps
we would tweak minislot-size to 2 = peak modem speed 11Mbps

6.4width 64qam = aggregate 26.7Mbps available
with default minislot-size 1 = peak modem speed 8.6Mbps
we would tweak minislot-size to 2 = peak modem speed 16Mbps

Oh, something else I thought of, with your MRTG there are other OIDs you can poll to get some useful stats. Rather than just plotting the bandwidth on the upstream, I find the OID that give utilisation % to be a pretty handy one. That way it doesnt matter if your upstreams have an assortment of different configs, or if they have auto-switch to lower width or modulations due to noise, you can always see how "full" the upstream is.

Here's some OIDs for you to play around with :

upstream_width = "1.3.6.1.2.1.10.127.1.1.2.1.3";
upstream_profile = "1.3.6.1.2.1.10.127.1.1.2.1.4";
unerrored = "1.3.6.1.2.1.10.127.1.1.4.1.2";
correctable = "1.3.6.1.2.1.10.127.1.1.4.1.3";
uncorrectable = "1.3.6.1.2.1.10.127.1.1.4.1.4";
snr = "1.3.6.1.2.1.10.127.1.1.4.1.5";
upstream_modems_tot = "1.3.6.1.4.1.9.9.116.1.4.1.1.3";
upstream_modems_reg = "1.3.6.1.4.1.9.9.116.1.4.1.1.4";
utilisation = "1.3.6.1.4.1.9.9.116.1.4.1.1.7";