Improve axis margin handling in QwtPlot

Fix #94

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Version 0.14.3
 
+- Fixed [Issue #94](https://github.com/PlotPyStack/PythonQwt/issues/94) - Different logarithmic scale behavior when compared to Qwt
 - Merged [PR #91](https://github.com/PlotPyStack/PythonQwt/pull/91): Fix: legend now showing up when enabled later - thanks to @nicoddemus
 - Removed `QwtPlotItem.setIcon` and `QwtPlotItem.icon` methods (introduced in 0.9.0 but not used in PythonQwt)
 

qwt/interval.py

@@ -396,18 +396,3 @@ def extend(self, value):
         if not self.isValid():
             return self
         return QwtInterval(min([value, self.__minValue]), max([value, self.__maxValue]))
-
-    def extend_fraction(self, value):
-        """
-        Extend the interval by a fraction of its width
-
-        :param float value: Fraction
-        :return: extended interval
-        """
-        if not self.isValid():
-            return self
-        return QwtInterval(
-            self.__minValue - value * self.width(),
-            self.__maxValue + value * self.width(),
-            self.__borderFlags,
-        )

qwt/plot.py

@@ -588,7 +588,7 @@ def axisStepSize(self, axisId):
     def axisMargin(self, axisId):
         """
         :param int axisId: Axis index
-        :return: Margin in % of the canvas size
+        :return: Relative margin of the axis, as a fraction of the full axis range
 
         .. seealso::
 
@@ -871,15 +871,17 @@ def setAxisMaxMajor(self, axisId, maxMajor):
 
     def setAxisMargin(self, axisId, margin):
         """
-        Set the margin of the scale widget
+        Set the relative margin of the axis, as a fraction of the full axis range
 
         :param int axisId: Axis index
-        :param float margin: Margin in % of the canvas size
+        :param float margin: Relative margin (float between 0 and 1)
 
         .. seealso::
 
             :py:meth:`axisMargin()`
         """
+        if not isinstance(margin, float) or margin < 0.0 or margin > 1.0:
+            raise ValueError("margin must be a float between 0 and 1")
         if self.axisValid(axisId):
             d = self.__axisData[axisId]
             if margin != d.margin:
@@ -946,16 +948,12 @@ def updateAxes(self):
             minValue = d.minValue
             maxValue = d.maxValue
             stepSize = d.stepSize
-            if d.margin is not None:
-                intv_i = intv[axisId].extend_fraction(d.margin)
-            else:
-                intv_i = intv[axisId]
-            if d.doAutoScale and intv_i.isValid():
+            if d.doAutoScale and intv[axisId].isValid():
                 d.isValid = False
-                minValue = intv_i.minValue()
-                maxValue = intv_i.maxValue()
+                minValue = intv[axisId].minValue()
+                maxValue = intv[axisId].maxValue()
                 minValue, maxValue, stepSize = d.scaleEngine.autoScale(
-                    d.maxMajor, minValue, maxValue, stepSize
+                    d.maxMajor, minValue, maxValue, stepSize, d.margin
                 )
             if not d.isValid:
                 d.scaleDiv = d.scaleEngine.divideScale(

qwt/scale_engine.py

@@ -37,8 +37,8 @@
 from qwt.transform import QwtLogTransform, QwtTransform
 
 DBL_MAX = sys.float_info.max
-LOG_MIN = 1.0e-100
-LOG_MAX = 1.0e100
+LOG_MIN = 1.0e-150
+LOG_MAX = 1.0e150
 
 
 def qwtLogInterval(base, interval):
@@ -198,14 +198,15 @@ def __init__(self, base=10):
         self.__data = QwtScaleEngine_PrivateData()
         self.setBase(base)
 
-    def autoScale(self, maxNumSteps, x1, x2, stepSize):
+    def autoScale(self, maxNumSteps, x1, x2, stepSize, relative_margin=0.0):
         """
         Align and divide an interval
 
         :param int maxNumSteps: Max. number of steps
         :param float x1: First limit of the interval (In/Out)
         :param float x2: Second limit of the interval (In/Out)
         :param float stepSize: Step size
+        :param float relative_margin: Margin as a fraction of the interval width
         :return: tuple (x1, x2, stepSize)
         """
         pass
@@ -473,20 +474,27 @@ class QwtLinearScaleEngine(QwtScaleEngine):
     def __init__(self, base=10):
         super(QwtLinearScaleEngine, self).__init__(base)
 
-    def autoScale(self, maxNumSteps, x1, x2, stepSize):
+    def autoScale(self, maxNumSteps, x1, x2, stepSize, relative_margin=0.0):
         """
         Align and divide an interval
 
         :param int maxNumSteps: Max. number of steps
         :param float x1: First limit of the interval (In/Out)
         :param float x2: Second limit of the interval (In/Out)
         :param float stepSize: Step size
+        :param float relative_margin: Margin as a fraction of the interval width
         :return: tuple (x1, x2, stepSize)
 
         .. seealso::
 
             :py:meth:`setAttribute()`
         """
+        # Apply the relative margin (fraction of the interval width) in linear space:
+        if relative_margin > 0.0:
+            margin = (x2 - x1) * relative_margin
+            x1 -= margin
+            x2 += margin
+
         interval = QwtInterval(x1, x2)
         interval = interval.normalized()
         interval.setMinValue(interval.minValue() - self.lowerMargin())
@@ -640,14 +648,15 @@ def __init__(self, base=10):
         super(QwtLogScaleEngine, self).__init__(base)
         self.setTransformation(QwtLogTransform())
 
-    def autoScale(self, maxNumSteps, x1, x2, stepSize):
+    def autoScale(self, maxNumSteps, x1, x2, stepSize, relative_margin=0.0):
         """
         Align and divide an interval
 
         :param int maxNumSteps: Max. number of steps
         :param float x1: First limit of the interval (In/Out)
         :param float x2: Second limit of the interval (In/Out)
         :param float stepSize: Step size
+        :param float relative_margin: Margin as a fraction of the interval width
         :return: tuple (x1, x2, stepSize)
 
         .. seealso::
@@ -657,11 +666,18 @@ def autoScale(self, maxNumSteps, x1, x2, stepSize):
         if x1 > x2:
             x1, x2 = x2, x1
         logBase = self.base()
+
+        # Apply the relative margin (fraction of the interval width) in logarithmic
+        # space, and convert back to linear space.
+        if relative_margin is not None:
+            log_margin = math.log(x2 / x1, logBase) * relative_margin
+            x1 /= math.pow(logBase, log_margin)
+            x2 *= math.pow(logBase, log_margin)
+
         interval = QwtInterval(
             x1 / math.pow(logBase, self.lowerMargin()),
             x2 * math.pow(logBase, self.upperMargin()),
         )
-        interval = interval.limited(LOG_MIN, LOG_MAX)
         if interval.maxValue() / interval.minValue() < logBase:
             linearScaler = QwtLinearScaleEngine()
             linearScaler.setAttributes(self.attributes())
@@ -674,7 +690,7 @@ def autoScale(self, maxNumSteps, x1, x2, stepSize):
             linearInterval = linearInterval.limited(LOG_MIN, LOG_MAX)
 
             if linearInterval.maxValue() / linearInterval.minValue() < logBase:
-                # The min / max interval is too short to be represented as a log scale. 
+                # The min / max interval is too short to be represented as a log scale.
                 # Set the step to 0, so that a new step is calculated and a linear scale is used.
                 stepSize = 0.0
                 return x1, x2, stepSize

qwt/tests/test_relativemargin.py

@@ -0,0 +1,54 @@
+# -*- coding: utf-8 -*-
+#
+# Licensed under the terms of the PyQwt License
+# Copyright (C) 2003-2009 Gerard Vermeulen, for the original PyQwt example
+# Copyright (c) 2015 Pierre Raybaut, for the PyQt5/PySide port and further
+# developments (e.g. ported to PythonQwt API)
+# (see LICENSE file for more details)
+
+SHOW = True  # Show test in GUI-based test launcher
+
+from qtpy import QtWidgets as QW
+from qtpy.QtCore import Qt
+
+import qwt
+from qwt.tests import utils
+
+
+class RelativeMarginDemo(QW.QWidget):
+    def __init__(self, *args):
+        QW.QWidget.__init__(self, *args)
+        layout = QW.QGridLayout(self)
+        x = [1, 2, 3, 4]
+        y = [1, 500, 1000, 1500]
+        for i_row, log_scale in enumerate((False, True)):
+            for i_col, relative_margin in enumerate((0.0, None, 0.2)):
+                plot = qwt.QwtPlot(self)
+                qwt.QwtPlotGrid.make(
+                    plot, color=Qt.lightGray, width=0, style=Qt.DotLine
+                )
+                def_margin = plot.axisMargin(qwt.QwtPlot.yLeft)
+                scale_str = "lin/lin" if not log_scale else "log/lin"
+                if relative_margin is None:
+                    margin_str = f"default ({def_margin*100:.0f}%)"
+                else:
+                    margin_str = f"{relative_margin*100:.0f}%"
+                plot.setTitle(f"{scale_str}, margin: {margin_str}")
+                if relative_margin is not None:
+                    plot.setAxisMargin(qwt.QwtPlot.yLeft, relative_margin)
+                    plot.setAxisMargin(qwt.QwtPlot.xBottom, relative_margin)
+                color = "red" if i_row == 0 else "blue"
+                qwt.QwtPlotCurve.make(x, y, "", plot, linecolor=color)
+                layout.addWidget(plot, i_row, i_col)
+                if log_scale:
+                    engine = qwt.QwtLogScaleEngine()
+                    plot.setAxisScaleEngine(qwt.QwtPlot.yLeft, engine)
+
+
+def test_relative_margin():
+    """Test relative margin."""
+    utils.test_widget(RelativeMarginDemo, size=(400, 300), options=False)
+
+
+if __name__ == "__main__":
+    test_relative_margin()