Sounds like something I did recently. In my case re-rendering the entire spectrogram was too slow. So I added a caching layer to get the speed I needed.

Qt Code: 
  1. class Waterfall : public QwtPlotSpectrogram
  2. {
  3. public:
  4. 	Waterfall(const QString &title = QString::null) :
  6. 	{
  7. 	}
  8.  
  9. 	virtual ~Waterfall()
  10. 	{
  11. 	}
  12.  
  13. protected:
  14. 	virtual QImage renderImage(const QwtScaleMap &xMap,
  15. 				   const QwtScaleMap &yMap,
  16. 				   const QRectF &rect) const
  17. 	{
  18. 		QRect rect = transform(xMap, yMap, area);
  19.  
  20. 		// I don't know why this adjustment is necessary, but
  21. 		// without it qwt plots a different area than we cache
  22. 		rect.adjust(-2, -2, -3, -3);
  23. 		QwtScaleMap xxMap = xMap;
  24. 		QwtScaleMap yyMap = yMap;
  25.  
  26. 		const QSize res(QwtPlotSpectrogram::rasterHint(area));
  27. 		if ( res.isValid() )
  28. 		{
  29. 			rect.setSize(rect.size().boundedTo(res));
  30. 		}
  31.  
  32. 		int px1 = rect.x();
  33. 		int px2 = rect.x() + rect.width();
  34. 		if ( xMap.p1() > xMap.p2() )
  35. 		qSwap(px1, px2);
  36.  
  37. 		double sx1 = area.x();
  38. 		double sx2 = area.x() + area.width();
  39. 		if ( xMap.s1() > xMap.s2() )
  40. 		qSwap(sx1, sx2);
  41.  
  42. 		int py1 = rect.y();
  43. 		int py2 = rect.y() + rect.height();
  44. 		if ( yMap.p1() > yMap.p2() )
  45. 		qSwap(py1, py2);
  46.  
  47. 		double sy1 = area.y();
  48. 		double sy2 = area.y() + area.height();
  49. 		if ( yMap.s1() > yMap.s2() )
  50. 		qSwap(sy1, sy2);
  51.  
  52. 		xxMap.setPaintInterval(px1, px2);
  53. 		xxMap.setScaleInterval(sx1, sx2);
  54. 		yyMap.setPaintInterval(py1, py2);
  55. 		yyMap.setScaleInterval(sy1, sy2);
  56.  
  57. 		// Redraw everything if the scale or size changed
  58. 		if (cachedImage_.isNull()
  59. 			|| xMap.pDist() != cachedXMap_.pDist()
  60. 			|| xMap.sDist() != cachedXMap_.sDist()
  61. 			|| yMap.pDist() != cachedYMap_.pDist()
  62. 			|| yMap.sDist() != cachedYMap_.sDist()
  63. 			|| area.width() != cachedArea_.width()
  64. 			|| area.height() != cachedArea_.height())
  65. 		{
  66. 			cachedImage_ = QwtPlotSpectrogram::renderImage(xMap, yMap, area);
  67. 			cachedXShift_ = 0.0;
  68. 			cachedYShift_ = 0.0;
  69. 		}
  70. 		else
  71. 		{
  72. 			QRect xRect, yRect;
  73.  
  74. 			// Shift the cached image to compensate for the area change
  75. 			// and calculate the x and y boxes to redraw
  76. 			//
  77. 			// 0
  78. 			//  +------------------------+  ---
  79. 			//  |         -yRect         |   |  yShift (negative)
  80. 			//  +---+----------------+---+  ---
  81. 			//  | - |                | + |   |
  82. 			//  | x |                | x |   |
  83. 			//  | R | Cached/shifted | R |   |  height - abs(yShift)
  84. 			//  | e |     image      | e |   |
  85. 			//  | c |                | c |   |
  86. 			//  | t |                | t |   |
  87. 			//  |   |                |   |   |
  88. 			//  +---+----------------+---+  ---
  89. 			//  |         +yRect         |   |  yShift (positive)
  90. 			//  +------------------------+  ---
  91. 			//
  92. 			//  |---|                |---|
  93. 			//  xShift (negative)    xShift (positive)
  94.  
  95. 			double xScale = xxMap.pDist() / xxMap.sDist();
  96. 			double yScale = yyMap.pDist() / yyMap.sDist();
  97. 			double xShift = -(cachedArea_.left() - area.left()) * xScale + cachedXShift_;
  98. 			double yShift = (cachedArea_.top() - area.top()) * yScale + cachedYShift_;
  99. 			cachedXShift_ = xShift - ceil(xShift);
  100. 			cachedYShift_ = yShift - ceil(yShift);
  101. 			xShift = ceil(xShift);
  102. 			yShift = ceil(yShift);
  103.  
  104. 			// Just redraw everything if the shift size is greater than the image
  105. 			if (abs(xShift) > rect.width() || abs(yShift) > rect.height())
  106. 			{
  107. 				cachedImage_ = QwtPlotSpectrogram::renderImage(xMap, yMap, area);
  108. 				cachedXShift_ = 0.0;
  109. 				cachedYShift_ = 0.0;
  110. 			}
  111. 			else
  112. 			{
  113. 				if (yShift < 0)
  114. 				{
  115. 					yRect.setCoords(rect.left(), rect.top(), rect.right(), rect.top() - yShift);
  116.  
  117. 					if (xShift < 0)
  118. 					{
  119. 						xRect.setCoords(rect.left(), yRect.bottom(), rect.left() - xShift, rect.bottom());
  120. 					}
  121. 					else if (xShift > 0)
  122. 					{
  123. 						xRect.setCoords(rect.right() - xShift, yRect.bottom(), rect.right(), rect.bottom());
  124. 					}
  125. 				}
  126. 				else if (yShift > 0)
  127. 				{
  128. 					yRect.setCoords(rect.left(), rect.bottom() - yShift, rect.right(), rect.bottom());
  129. 					if (xShift < 0)
  130. 					{
  131. 						xRect.setCoords(rect.left(), rect.top(), rect.left() - xShift, yRect.top());
  132. 					}
  133. 					else if (xShift > 0)
  134. 					{
  135. 						xRect.setCoords(rect.right() - xShift, rect.top(), rect.right(), yRect.top());
  136. 					}
  137. 				}
  138. 				else
  139. 				{
  140. 					if (xShift < 0)
  141. 					{
  142. 						xRect.setCoords(rect.left(), rect.top(), rect.left() - xShift, rect.bottom());
  143. 					}
  144. 					else if (xShift > 0)
  145. 					{
  146. 						xRect.setCoords(rect.right() - xShift, rect.top(), rect.right(), rect.bottom());
  147. 					}
  148. 				}
  149.  
  150. 				// Without this re-adjustment the scale is off by 1 pixel in each direction
  151. 				rect.adjust(0, 0, 1, 1);
  152. 				if (!yRect.isNull() || !xRect.isNull())
  153. 				{
  154. 					QImage newImage(rect.size(), cachedImage_.format());
  155. 					QPainter painter(&newImage);
  156. 					newImage.fill(0);
  157.  
  158. 					if (!yRect.isNull())
  159. 					{
  160. 						QImage Image;
  161. 						QRectF sArea = invTransform(xxMap, yyMap, yRect);
  162. 						Image = QwtPlotSpectrogram::renderImage(xxMap, yyMap, sArea);
  163. 						painter.drawImage(yRect, Image);
  164. 					}
  165.  
  166. 					if (!xRect.isNull())
  167. 					{
  168. 						QImage image;
  169. 						QRectF sArea = invTransform(xxMap, yyMap, xRect);
  170. 						image = QwtPlotSpectrogram::renderImage(xxMap, yyMap, sArea);
  171. 						painter.drawImage(xRect, image);
  172. 					}
  173.  
  174. 					painter.drawImage(rect.translated(-xShift, -yShift), cachedImage_, rect);
  175. 					cachedImage_ = newImage;
  176. 				}
  177. 			}
  178. 		}
  179.  
  180. 		cachedXMap_ = xMap;
  181. 		cachedYMap_ = yMap;
  182. 		cachedArea_ = area;
  183. 		cachedRect_ = rect;
  184.  
  185. 		return cachedImage_;
  186. 	}
  187.  
  188. private:
  189. 	mutable QImage cachedImage_;
  190. 	mutable QwtScaleMap cachedXMap_;
  191. 	mutable QwtScaleMap cachedYMap_;
  192. 	mutable QRectF cachedArea_;
  193. 	mutable QRect cachedRect_;
  194. 	mutable double cachedXShift_;
  195. 	mutable double cachedYShift_;
  196. };
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If you need it pixel perfect you might have to fiddle with it a bit. I think it still had an off by one problem in the cached image.