diff --git a/src/graph_gui.cpp b/src/graph_gui.cpp
index 5c15eaf53c..4e11236520 100644
--- a/src/graph_gui.cpp
+++ b/src/graph_gui.cpp
@@ -199,7 +199,7 @@ protected:
 	/* These values are used if the graph is being plotted against values
 	 * rather than the dates specified by month and year. */
 	uint16_t x_values_start;
-	int16_t x_values_increment;
+	uint16_t x_values_increment;
 
 	StringID format_str_y_axis;
 	uint8_t colours[GRAPH_MAX_DATASETS];
@@ -1429,8 +1429,8 @@ struct IndustryProductionGraphWindow : BaseGraphWindow {
 		this->num_on_x_axis = GRAPH_NUM_MONTHS;
 		this->num_vert_lines = GRAPH_NUM_MONTHS;
 		this->month_increment = 1;
-		this->x_values_start = GRAPH_NUM_MONTHS;
-		this->x_values_increment = -ECONOMY_MONTH_MINUTES;
+		this->x_values_start = ECONOMY_MONTH_MINUTES;
+		this->x_values_increment = ECONOMY_MONTH_MINUTES;
 		this->draw_dates = !TimerGameEconomy::UsingWallclockUnits();
 
 		this->CreateNestedTree();