How to fix:
admin/statistics_function.php line: 3055

Change:
$pdf->Image($tempdir."/".$cachefilename, 5, 70, 200, 200, '', $homeurl."/admin.php?sid=$surveyid", 'B', true, 150,'',false,false,0,true);

to->

$pdf->Image($tempdir."/".$cachefilename, 25, 70, 160, 200, '', $homeurl."/admin.php?sid=$surveyid", 'B', true, 150,'',false,false,0,true);

25 putes the picture towards the centre.
160 is half of 320 which is the original width size.

Has tried this originally in a linux webhosting enviroment with same problems.

Please attach an export of your survey. Thank you.

I can't see any distorted graphs in your PDF file. Maybe your viewer really has a bug?

Tried Foxit Reader and there where no distortion!

So, apparently Adobe Reader 9.3.3 got a bug. Or TCPDF, doesn't render it properly?

Well, update Adobe Reader to 9.4 first.

Did so - same problem.

I now installed Adobre Reader 10 and your file looks fine here too.
I think something is wrong with your Adobe installation.

Tried on a diffrent windows machine with latest updates. Same problem
Tried even on a third diffrent machine, with Ubuntu 10.10 with latest updates using Adobe Reader 9.4. Same problem.
I have also tried to open the PDF in Acrobat X Pro. Same problem.

It must be a bug withtin the Adobe PDF rendering or at TCPDF?

Can you attach a screenshot of how you see the attached PFD file distorted?

When viewing the file do the following in Adobe ACrobat Reader.
GO to Edit/Preferences. Choose Page Display and on the right set set Resoltion ti use the System setting. Also set Rendering to 'For Screen display'.
Save and notice that it looks okay now - so Adobe is to blame.

For that much support & effort you should seriously consider a donation to the LimeSurvey project.

Released in latest 1.90+ version.