When you try to display a burndown with a duration of 1 day, the following notices and error are raised:
PHP Notice: Undefined offset: 26 in /usr/share/jpgraph/jpgraph_date.php on line 312
PHP Stack trace:
PHP 1. {main}() /usr/share/tuleap/plugins/tracker/www/index.php:0
PHP 2. TrackerManager->process() /usr/share/tuleap/plugins/tracker/www/index.php:33
PHP 3. TrackerManager->processSubElement() /usr/share/tuleap/plugins/tracker/include/Tracker/TrackerManager.class.php:124
PHP 4. Tracker_FormElement_Field_Burndown->process() /usr/share/tuleap/plugins/tracker/include/Tracker/TrackerManager.class.php:107
PHP 5. Tracker_FormElement_Field_Burndown->fetchBurndownImage() /usr/share/tuleap/plugins/tracker/include/Tracker/FormElement/Tracker_FormElement_Field_Burndown.class.php:141
PHP 6. Tracker_Chart_Burndown->display() /usr/share/tuleap/plugins/tracker/include/Tracker/FormElement/Tracker_FormElement_Field_Burndown.class.php:160
PHP 7. Chart->stroke() /usr/share/tuleap/plugins/tracker/include/Tracker/Chart/Burndown.class.php:204
PHP 8. Chart->__call() /usr/share/tuleap/plugins/tracker/include/Tracker/Chart/Burndown.class.php:204
PHP 9. call_user_func_array() /usr/share/tuleap/src/common/chart/Chart.class.php:160
PHP 10. Graph->Stroke() /usr/share/tuleap/src/common/chart/Chart.class.php:160
PHP 11. DateScale->AutoScale() /usr/share/jpgraph/jpgraph.php:1745
PHP 12. DateScale->DoDateAutoScale() /usr/share/jpgraph/jpgraph_date.php:418
JpGraph error for graph "": Minor or major step size is 0. Check that you haven't got an accidental SetTextTicks(0) in your code. If this is not the case you might have stumbled upon a bug in JpGraph. Please report this and if possible include the data that caused the problem

After some quick investigations, it seems this is due to a bug inside the JPGraph library.