The following error is obtained when trying to generate pie chart from a survey in test environment (with a complex survey-4 groups with one question each containing up to 24 answers using labetset to choose from 0 to 5):
"PHP Notice: Uninitialized string offset: 0 in C:\Sources\limesurvey\classes\pchart\pchart\pData.class on line 127 PHP Notice: Uninitialized string offset: 1 in C:\Sources\limesurvey\classes\pchart\pchart\pData.class on line 127..."

Steps I performed - Test1:

1.I clicked "view statistics" and checked "All questions" and clicked View stats (at the bottom of the page)-> Stats are displayed in a new page and can be exported.
2.Back to the "View statistics" main page, I checked "Display chart", selected the first question and clicked the "View stats" (at the bottom of the page)->http 500 error is displayed
3.Perform same as in step 2 but first clean all the .png files from the tmp folder ->a new .png file is generated with a pie chart in it.

Test 2:
This error occurred at first in my surveys with several questions so I tried to reproduce it in a new and simple survey:
1.I created a closed survey with only one group and one question
2.I created a token and sent email to one user
3.I activated the survey
4.I answered the simple survey using the link from the email
5.I accessed the answers page
6.I listed the results saved -> Results are dispolayed
7.I clicked "view statistics" and checked "All questions" and clicked View stats (at the bottom of the page)-> Stats are displayed in a new page and can be exported.
8.Back to the "View statistics" main page, I checked "Display chart", selected the question and clicked the "View stats" (at the bottom of the page)->http 500 error is presented
9.Back to the "View statistics" main page, I checked "Display chart", selected the question and clicked the "View stats" (at the bottom of the page) + previoulsy deleted all .png files from tmp folder ->same http 500 error is presented + file .png is generated in tmp folder (eb258795485f465f563377a23799d636.png)
10.Same as test 9 but selecting several answers (with different values selected by the user)+ previoulsy deleted all .png files from tmp folder -> error on page + three .png files generated

Notes:
I gave all the permissions to the tmp folder.
It seems that the application is able to generate the image files but it is not able to present them.