搞了半天，总算是把直方图给搞明白了，总结了一下，对直方图生成再拉伸，代码如下：

    int size=256; 
    float range[]={0,255}; 
    float* ranges[]={range}; 
    CvHistogram* hist=cvCreateHist(1,&size,CV_HIST_ARRAY,ranges,1); 
    cvCalcHist(&scale1,hist,0,NULL); 
    float max=0; 
    cvGetMinMaxHistValue(hist,NULL,&max,NULL,NULL); 
    double max_indensity=0;
    cvMinMaxLoc(scale1,NULL,&max_indensity,NULL,NULL);

    IplImage* dst=cvCreateImage(cvSize(256,300),8,3); 
    cvSet(dst,cvScalarAll(255),0); 
    double bin_width=(double)dst->width/max_indensity; 
    double bin_unith=(double)dst->height/max; 
    for(int i=0;i<256;i++) 
    { 
        CvPoint p0=cvPoint(i*bin_width,dst->height);  //左下角点
        CvPoint p1=cvPoint((i+1)*bin_width,dst->height-cvGetReal1D(hist->bins,i)*bin_unith);  //右上角点
        cvRectangle(dst,p0,p1,cvScalar(0,255，0),-1,8,0); 
    }