JPGRAPH le fait sans autre programmation que ça :
$Graphe = new GanttGraph($this->_LargeurImage,$this->_HauteurImage,"auto");
$Graphe->SetBox();
$Graphe->SetShadow();
$Graphe->ShowHeaders(GANTT_HMONTH | GANTT_HYEAR); # Show day, week and month scale
$Graphe->scale->month->SetStyle(MONTHSTYLE_SHORTNAME); # Use the short name of the month together with a 4 digit year
$Graphe->scale->month->SetFontColor("white");
$Graphe->scale->month->SetBackgroundColor("blue");
$Graphe->SetLabelVMarginFactor(1); # 0 % vertical label margin
$Activite = new GanttBar($Compteur,$Vague,$DatesDebut[$Compteur],$DatesFin[$Compteur],"(".$Vague.")");
$Activite->SetPattern(BAND_RDIAG,"yellow"); # Yellow diagonal line pattern
$Activite->SetFillColor($Couleurs[$Compteur]);
$Activite->SetHeight(10); # Set absolute height
$Graphe->Add($Activite); # Finally add the bar to the graph