Bresenham algorithm

I use bresenhams line algorithm to draw a line.Sometimes when the slope is close to 0.5 the line isn't drawing correctly.Some parts of the line are thicker than other or the line that is drawn looks like a triangle.The following code is for the first and fourth octant.
Why this is happening?

Code:

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public void        draw_line(Graphics g,int x1,int y1,int xn,int yn)
        {
                int error,dx,dy,y,x;
               
       

                dx=xn-x1;
                dy=yn-y1;
               
                y=y1;
                x=x1;
                //the octant of the line
                if(slope==1||slope==4)
                {
                        error=-dx/2;
                for( x=x1;x<=xn;x++)
                { 
                        PutPixel(g,x,y);
                        error=error+dy;
                if(error>=0)
                {y++;
                error=error-dx;}}}

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