我们先看下一段代码，这行代码是说在轴上对应的值转化成它对应的像素值。
public float Transform( double x )
{
// Must take into account Log, and Reverse Axes
double denom = ( _maxLinTemp - _minLinTemp );
double ratio;
if ( denom > 1e-100 )
ratio = ( Linearize( x ) - _minLinTemp ) / denom;
else
ratio = 0;
if ( _isReverse == ( _ownerAxis is XAxis || _ownerAxis is X2Axis ) )
return (float) ( _maxPix - ( _maxPix - _minPix ) * ratio );
else
return (float) ( _minPix + ( _maxPix - _minPix ) * ratio );
}
这段代码很好理解，我们假设这是X轴，轴没有反转，X轴上的坐标最小值minLinTemp,对应像素是minpix,坐标最大值是maxLinTemp,对应像素是maxpix，现有一点在x轴的坐标是xlin,那么它对应的像素是xpix,它们满足这样一个关系，(xlin-minLinTemp)/(maxLinTemp-minLinTemp)=(xpix-minpix)/(maxpix-minpix),根据这个关系我们能得到对应的xpix的关系。如果是Y轴，因为我们坐标系上的Y轴和在设备上的坐标系是反着来的，那么它们的关系要改着这样了,(xlin-minLinTemp)/(maxLinTemp-minLinTemp)=(maxpix-xpix)/(maxpix-minpix).
然后我们查看一下Chart图上的元素是如何绘制的，我们这里看线条的绘制过程。线条的元素LineItem里会封装一个Line的对象，这个对象主要是负责具体画什么样的线，如直线，基数样条等，我们看一下普通的基数样条的绘制代码。
public void DrawCurve( Graphics g, GraphPane pane,
CurveItem curve, float scaleFactor )
{
Line source = this;
if ( curve.IsSelected )
source = Selection.Line;
// switch to int to optimize drawing speed (per Dale-a-b)
int tmpX, tmpY,
lastX = int.MaxValue,
lastY = int.MaxValue;
double curX, curY, lowVal;
PointPair curPt, lastPt = new PointPair();
bool lastBad = true;
IPointList points = curve.Points;
ValueHandler valueHandler = new ValueHandler( pane, false );
Axis yAxis = curve.GetYAxis( pane );
Axis xAxis = curve.GetXAxis( pane );
bool xIsLog = xAxis._scale.IsLog;
bool yIsLog = yAxis._scale.IsLog;
// switch to int to optimize drawing speed (per Dale-a-b)
int minX = (int)pane.Chart.Rect.Left;
int maxX = (int)pane.Chart.Rect.Right;
int minY = (int)pane.Chart.Rect.Top;
int maxY = (int)pane.Chart.Rect.Bottom;
using ( Pen pen = source.GetPen( pane, scaleFactor ) )
{
if ( points != null && !_color.IsEmpty && this.IsVisible )
{
//bool lastOut = false;
bool isOut;
bool isOptDraw = _isOptimizedDraw && points.Count > 1000;
// (Dale-a-b) we'll set an element to true when it has been drawn
bool[,] isPixelDrawn = null;
if ( isOptDraw )
isPixelDrawn = new bool[maxX + 1, maxY + 1];
// Loop over each point in the curve
for ( int i = 0; i < points.Count; i++ )
{
curPt = points[i];
if ( pane.LineType == LineType.Stack )
{
if ( !valueHandler.GetValues( curve, i, out curX, out lowVal, out curY ) )
{
curX = PointPair.Missing;
curY = PointPair.Missing;
}
}
else
{
curX = curPt.X;
curY = curPt.Y;
}
// Any value set to double max is invalid and should be skipped
// This is used for calculated values that are out of range, divide
// by zero, etc.
// Also, any value <= zero on a log scale is invalid
if ( curX == PointPair.Missing ||
curY == PointPair.Missing ||
System.Double.IsNaN( curX ) ||
System.Double.IsNaN( curY ) ||
System.Double.IsInfinity( curX ) ||
System.Double.IsInfinity( curY ) ||
( xIsLog && curX <= 0.0 ) ||
( yIsLog && curY <= 0.0 ) )
{
// If the point is invalid, then make a linebreak only if IsIgnoreMissing is false
// LastX and LastY are always the last valid point, so this works out
lastBad = lastBad || !pane.IsIgnoreMissing;
isOut = true;
}
else
{
// Transform the current point from user scale units to
// screen coordinates
tmpX = (int) xAxis.Scale.Transform( curve.IsOverrideOrdinal, i, curX );
tmpY = (int) yAxis.Scale.Transform( curve.IsOverrideOrdinal, i, curY );
// Maintain an array of "used" pixel locations to avoid duplicate drawing operations
// contributed by Dale-a-b
if ( isOptDraw && tmpX >= minX && tmpX <= maxX &&
tmpY >= minY && tmpY <= maxY ) // guard against the zoom-in case
{
if ( isPixelDrawn[tmpX, tmpY] )
continue;
isPixelDrawn[tmpX, tmpY] = true;
}
isOut = ( tmpX < minX && lastX < minX ) || ( tmpX > maxX && lastX > maxX ) ||
( tmpY < minY && lastY < minY ) || ( tmpY > maxY && lastY > maxY );
if ( !lastBad )
{
try
{
// GDI+ plots the data wrong and/or throws an exception for
// outrageous coordinates, so we do a sanity check here
if ( lastX > 5000000 || lastX < -5000000 ||
lastY > 5000000 || lastY < -5000000 ||
tmpX > 5000000 || tmpX < -5000000 ||
tmpY > 5000000 || tmpY < -5000000 )
InterpolatePoint( g, pane, curve, lastPt, scaleFactor, pen,
lastX, lastY, tmpX, tmpY );
else if ( !isOut )
{
if ( !curve.IsSelected && this._gradientFill.IsGradientValueType )
{
using ( Pen tPen = GetPen( pane, scaleFactor, lastPt ) )
{
if ( this.StepType == StepType.NonStep )
{
g.DrawLine( tPen, lastX, lastY, tmpX, tmpY );
}
else if ( this.StepType == StepType.ForwardStep )
{
g.DrawLine( tPen, lastX, lastY, tmpX, lastY );
g.DrawLine( tPen, tmpX, lastY, tmpX, tmpY );
}
else if ( this.StepType == StepType.RearwardStep )
{
g.DrawLine( tPen, lastX, lastY, lastX, tmpY );
g.DrawLine( tPen, lastX, tmpY, tmpX, tmpY );
}
else if ( this.StepType == StepType.ForwardSegment )
{
g.DrawLine( tPen, lastX, lastY, tmpX, lastY );
}
else
{
g.DrawLine( tPen, lastX, tmpY, tmpX, tmpY );
}
}
}
else
{
if ( this.StepType == StepType.NonStep )
{
g.DrawLine( pen, lastX, lastY, tmpX, tmpY );
}
else if ( this.StepType == StepType.ForwardStep )
{
g.DrawLine( pen, lastX, lastY, tmpX, lastY );
g.DrawLine( pen, tmpX, lastY, tmpX, tmpY );
}
else if ( this.StepType == StepType.RearwardStep )
{
g.DrawLine( pen, lastX, lastY, lastX, tmpY );
g.DrawLine( pen, lastX, tmpY, tmpX, tmpY );
}
else if ( this.StepType == StepType.ForwardSegment )
{
g.DrawLine( pen, lastX, lastY, tmpX, lastY );
}
else if ( this.StepType == StepType.RearwardSegment )
{
g.DrawLine( pen, lastX, tmpY, tmpX, tmpY );
}
}
}
}
catch
{
InterpolatePoint( g, pane, curve, lastPt, scaleFactor, pen,
lastX, lastY, tmpX, tmpY );
}
}
lastPt = curPt;
lastX = tmpX;
lastY = tmpY;
lastBad = false;
//lastOut = isOut;
}
}
}
}
}
代码有些多，但是过程还是很清晰的，前面一部分代码都是来获取相关数据，然后遍历对应CurveItem里的IPointList数据，把每个Point数据(是我们看到的数据)对应当前CurveItem里的X轴与Y轴上的像素值(给机器来看的)，这里用到的转换就是上面所说的那部分代码，根据我们选择StepType不同来绘制每二点决定的线。这样就绘制出了我们要的线条，别的元素具体上不同，但是都是这种逻辑。其实如果要多了解这一方面的逻辑，可以看下有关坐标系统和转换的知识。
PS:读源码有个地方要注意，不要只是跟着代码往下来，看一会不是分支过多就是跑题了，看的时候你要自己跟着想，如果是我来实现这一功能，应该怎么来实现，然后和看的代码比对。