diff --git a/bibliography.bib b/bibliography.bib
index 9b93415..0e2e9c5 100644
--- a/bibliography.bib
+++ b/bibliography.bib
@@ -3051,6 +3051,16 @@
   file = {/home/charlotte/sync/Zotero/storage/8A8ASDV2/skulpt.org.html}
 }
 
+@mastersthesis{selsTESTedProgrammeertaalonafhankelijkTesten2021,
+  title = {{TESTed: programmeertaal-onafhankelijk testen van oplossingen voor programmeeroefeningen: Eenvoudig oefeningen opstellen met een DSL}},
+  author = {Sels, Boris and Dawyndt, Peter and Mesuere, Bart and Strijbol, Niko and Van Petegem, Charlotte},
+  year = {2021},
+  url = {http://lib.ugent.be/catalog/rug01:003008250},
+  langid = {und},
+  school = {Ghent University},
+  file = {/home/charlotte/sync/Zotero/storage/34VD6QYR/Sels et al. - 2021 - TESTed programmeertaal-onafhankelijk testen van o.pdf}
+}
+
 @article{shanaIdentifyingKeyPerformance2011,
   title = {Identifying Key Performance Indicators and Predicting the Result from Student Data},
   author = {Shana, J. and Venkatachalam, T.},
@@ -3553,6 +3563,19 @@
   file = {/home/charlotte/sync/Zotero/storage/PTM64G3B/Watson and Li - 2014 - Failure rates in introductory programming revisite.pdf}
 }
 
+@misc{wickhamGgplot2CreateElegant2023,
+  title = {Ggplot2: {{Create Elegant Data Visualisations Using}} the {{Grammar}} of {{Graphics}}},
+  shorttitle = {Ggplot2},
+  author = {Wickham, Hadley and Chang, Winston and Henry, Lionel and Pedersen, Thomas Lin and Takahashi, Kohske and Wilke, Claus and Woo, Kara and Yutani, Hiroaki and Dunnington, Dewey and Posit and PBC},
+  year = {2023},
+  month = oct,
+  url = {https://cran.r-project.org/web/packages/ggplot2/index.html},
+  urldate = {2023-12-08},
+  abstract = {A system for 'declaratively' creating graphics, based on "The Grammar of Graphics". You provide the data, tell 'ggplot2' how to map variables to aesthetics, what graphical primitives to use, and it takes care of the details.},
+  copyright = {MIT + file LICENSE},
+  keywords = {Phylogenetics,Spatial,TeachingStatistics}
+}
+
 @book{wiegersCreatingSoftwareEngineering1996,
   title = {Creating a Software Engineering Culture},
   author = {Wiegers, Karl Eugene},