diff --git a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/StretchesFunction.java b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/StretchesFunction.java
index 506f298f4..818cce830 100644
--- a/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/StretchesFunction.java
+++ b/navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/StretchesFunction.java
@@ -82,9 +82,8 @@ public class StretchesFunction extends AssignmentFunction {
 
         public static double[] getHoursPointsFor(int totalHours,
                 List<Interval> intervalsDefinedByStreches) {
-            double[] result = new double[intervalsDefinedByStreches.size() + 1];
-            int i = 1;
-            result[0] = 0;
+            double[] result = new double[intervalsDefinedByStreches.size()];
+            int i = 0;
             int accumulated = 0;
             for (Interval each : intervalsDefinedByStreches) {
                 accumulated += each.getHoursFor(totalHours);
@@ -95,9 +94,8 @@ public class StretchesFunction extends AssignmentFunction {
 
         public static double[] getDayPointsFor(LocalDate start,
                 List<Interval> intervalsDefinedByStreches) {
-            double[] result = new double[intervalsDefinedByStreches.size() + 1];
-            result[0] = 0;
-            int i = 1;
+            double[] result = new double[intervalsDefinedByStreches.size()];
+            int i = 0;
             for (Interval each : intervalsDefinedByStreches) {
                 result[i++] = Days.daysBetween(start, each.getEnd()).getDays();
             }
@@ -429,16 +427,6 @@ public class StretchesFunction extends AssignmentFunction {
         return getStretchesPlusConsolidated().size() >= 2;
     }
 
-    public boolean checkFirstIntervalIsPosteriorToDate(LocalDate date) {
-        List<Interval> intervals = StretchesFunction.intervalsFor(
-                resourceAllocation, getStretchesPlusConsolidated());
-        if (intervals.isEmpty()) {
-            return false;
-        }
-        Interval first = intervals.get(0);
-        return first.getEnd().compareTo(date) > 0;
-    }
-
     public boolean isInterpolated() {
         return getDesiredType().equals(StretchesFunctionTypeEnum.INTERPOLATED);
     }
diff --git a/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/GraphicForStreches.java b/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/GraphicForStreches.java
index d405c217f..236e59aeb 100644
--- a/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/GraphicForStreches.java
+++ b/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/GraphicForStreches.java
@@ -164,14 +164,13 @@ public abstract class GraphicForStreches implements IGraphicGenerator {
 
         @Override
         public boolean areChartsEnabled(IStretchesFunctionModel model) {
-            return canComputeChartFrom(model.getResourceAllocation(),
-                    model.getStretchesPlusConsolidated());
+            return canComputeChartFrom(model.getStretchesPlusConsolidated());
         }
 
         @Override
         protected XYModel getAccumulatedHoursChartData(List<Stretch> stretches,
                 ResourceAllocation<?> allocation, BigDecimal taskHours) {
-            if (!canComputeChartFrom(allocation, stretches)) {
+            if (!canComputeChartFrom(stretches)) {
                 return new SimpleXYModel();
             }
             int[] hoursForEachDayUsingSplines = hoursForEachDayInterpolatedUsingSplines(
@@ -184,7 +183,7 @@ public abstract class GraphicForStreches implements IGraphicGenerator {
         protected XYModel getDedicationChart(List<Stretch> stretches,
                 ResourceAllocation<?> allocation, BigDecimal totalHours,
                 BaseCalendar taskCalendar) {
-            if (!canComputeChartFrom(allocation, stretches)) {
+            if (!canComputeChartFrom(stretches)) {
                 return new SimpleXYModel();
             }
             int[] dataForChart = hoursForEachDayInterpolatedUsingSplines(
@@ -192,10 +191,8 @@ public abstract class GraphicForStreches implements IGraphicGenerator {
             return createModelFrom(allocation.getStartDate(), dataForChart);
         }
 
-        private boolean canComputeChartFrom(ResourceAllocation<?> allocation,
-                List<Stretch> stretches) {
-            return StretchesFunctionModel.areValidForInterpolation(allocation,
-                    stretches);
+        private boolean canComputeChartFrom(List<Stretch> stretches) {
+            return StretchesFunctionModel.areValidForInterpolation(stretches);
         }
 
         private int[] hoursForEachDayInterpolatedUsingSplines(
diff --git a/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StretchesFunctionModel.java b/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StretchesFunctionModel.java
index 32196a582..aec814c8b 100644
--- a/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StretchesFunctionModel.java
+++ b/navalplanner-webapp/src/main/java/org/navalplanner/web/planner/allocation/streches/StretchesFunctionModel.java
@@ -27,6 +27,7 @@ import java.math.BigDecimal;
 import java.text.DateFormat;
 import java.util.Collections;
 import java.util.Date;
+import java.util.Iterator;
 import java.util.List;
 
 import org.joda.time.LocalDate;
@@ -39,7 +40,6 @@ import org.navalplanner.business.planner.entities.AssignmentFunction;
 import org.navalplanner.business.planner.entities.ResourceAllocation;
 import org.navalplanner.business.planner.entities.Stretch;
 import org.navalplanner.business.planner.entities.StretchesFunction;
-import org.navalplanner.business.planner.entities.StretchesFunction.Interval;
 import org.navalplanner.business.planner.entities.StretchesFunctionTypeEnum;
 import org.navalplanner.business.planner.entities.Task;
 import org.springframework.beans.factory.annotation.Autowired;
@@ -182,10 +182,6 @@ public class StretchesFunctionModel implements IStretchesFunctionModel {
                     throw new ValidationException(
                             _("There must be at least 2 stretches for doing interpolation"));
                 }
-                if (!stretchesFunction.checkFirstIntervalIsPosteriorToDate(getTaskStartDate())) {
-                    throw new ValidationException(
-                            _("The first stretch must be after the first day for doing interpolation"));
-                }
             }
             if (originalStretchesFunction != null) {
                 originalStretchesFunction
@@ -197,26 +193,36 @@ public class StretchesFunctionModel implements IStretchesFunctionModel {
         }
     }
 
-    public static boolean areValidForInterpolation(
-            ResourceAllocation<?> resourceAllocation, List<Stretch> stretches) {
-        return atLeastTwoStreches(stretches)
-                && theFirstIntervalIsPosteriorToFirstDay(resourceAllocation,
-                        stretches);
+    public static boolean areValidForInterpolation(List<Stretch> stretches) {
+        return atLeastThreeStreches(stretches)
+                && areStretchesSortedAndWithIncrements(stretches);
     }
 
-    private static boolean atLeastTwoStreches(List<Stretch> stretches) {
-        return stretches.size() >= 2;
+    private static boolean atLeastThreeStreches(List<Stretch> stretches) {
+        return stretches.size() >= 3;
     }
 
-    private static boolean theFirstIntervalIsPosteriorToFirstDay(
-            ResourceAllocation<?> resourceAllocation, List<Stretch> stretches) {
-        List<Interval> intervals = StretchesFunction.intervalsFor(
-                resourceAllocation, stretches);
-        if (intervals.isEmpty()) {
+    private static boolean areStretchesSortedAndWithIncrements(
+            List<Stretch> stretches) {
+        if (stretches.isEmpty()) {
             return false;
         }
-        Interval first = intervals.get(0);
-        return first.getEnd().compareTo(resourceAllocation.getStartDate()) > 0;
+
+        Iterator<Stretch> iterator = stretches.iterator();
+        Stretch previous = iterator.next();
+        while (iterator.hasNext()) {
+            Stretch current = iterator.next();
+            if (current.getLengthPercentage().compareTo(
+                    previous.getLengthPercentage()) <= 0) {
+                return false;
+            }
+            if (current.getAmountWorkPercentage().compareTo(
+                    previous.getAmountWorkPercentage()) <= 0) {
+                return false;
+            }
+            previous = current;
+        }
+        return true;
     }
 
     @Override