on axes plot the summed LFP contributions
    
    args:
    ::
        
        ax : matplotlib.axes.AxesSubplot object
        fname : str/np.ndarray, path to h5 file or ndim=2 numpy.ndarray 
        unit : str, scalebar unit
        scaling_factor : float, scaling factor (e.g. to scale 10% data set up)
        ylabels : bool, show labels on y-axis
        scalebar : bool, show scalebar in plot
        vlimround : None/float, override autoscaling of data and scalebar
        T : list, [tstart, tstop], which timeinterval
        ylim : list of floats, see plt.gca().set_ylim
        color : str/colorspec tuple, color of shown lines
        fancy : bool,
        label : str, line labels
        rasterized : bool, rasterize line plots if true
        kwargs : additional keyword arguments passed to ax.plot()
    
    
    returns:
    ::
        
        vlimround : float, scalebar scaling factor, i.e., to match up plots

def plot_signal_sum(ax, params, fname='LFPsum.h5', unit='mV', scaling_factor=1.,
                    ylabels=True, scalebar=True, vlimround=None,
                    T=[800, 1000], ylim=[-1500, 0], color='k', fancy=False,
                    label='', transient=200, clip_on=False, rasterized=True,
                    **kwargs):
    '''
    on axes plot the summed LFP contributions
    
    args:
    ::
        
        ax : matplotlib.axes.AxesSubplot object
        fname : str/np.ndarray, path to h5 file or ndim=2 numpy.ndarray 
        unit : str, scalebar unit
        scaling_factor : float, scaling factor (e.g. to scale 10% data set up)
        ylabels : bool, show labels on y-axis
        scalebar : bool, show scalebar in plot
        vlimround : None/float, override autoscaling of data and scalebar
        T : list, [tstart, tstop], which timeinterval
        ylim : list of floats, see plt.gca().set_ylim
        color : str/colorspec tuple, color of shown lines
        fancy : bool,
        label : str, line labels
        rasterized : bool, rasterize line plots if true
        kwargs : additional keyword arguments passed to ax.plot()
    
    
    returns:
    ::
        
        vlimround : float, scalebar scaling factor, i.e., to match up plots
    
    '''

    if type(fname) == str and os.path.isfile(fname):
        f = h5py.File(fname)
        #load data
        data = f['data'].value
            
        tvec = np.arange(data.shape[1]) * 1000. / f['srate'].value    

        #for mean subtraction
        datameanaxis1 = f['data'].value[:, tvec >= transient].mean(axis=1)
        
        #close dataset
        f.close()
    elif type(fname) == np.ndarray and fname.ndim==2:
        data = fname
        tvec = np.arange(data.shape[1]) * params.dt_output
        datameanaxis1 = data[:, tvec >= transient].mean(axis=1)
    else:
        raise Exception, 'type(fname)={} not str or numpy.ndarray'.format(type(fname))

    # slice
    slica = (tvec <= T[1]) & (tvec >= T[0])
    data = data[:,slica]
    
    #subtract mean in each channel
    #dataT = data.T - data.mean(axis=1)
    dataT = data.T - datameanaxis1
    data = dataT.T

    # normalize
    data = data*scaling_factor
    
    zvec = np.r_[params.electrodeParams['z']]
    zvec = np.r_[zvec, zvec[-1] + np.diff(zvec)[-1]]
    vlim = abs(data).max()
    if vlimround is None:
        vlimround = 2.**np.round(np.log2(vlim))
    else:
        pass
    yticklabels=[]
    yticks = []
    
    if fancy:
        colors=phlp.get_colors(data.shape[0])
    else:
        colors = [color]*data.shape[0]
    
    for i, z in enumerate(params.electrodeParams['z']):
        if i == 0:
            ax.plot(tvec[slica], data[i] * 100 / vlimround + z,
                    color=colors[i], rasterized=rasterized, label=label,
                    clip_on=clip_on, **kwargs)
        else: 
            ax.plot(tvec[slica], data[i] * 100 / vlimround + z,
                    color=colors[i], rasterized=rasterized, clip_on=clip_on,
                    **kwargs)
        yticklabels.append('ch. %i' % (i+1))
        yticks.append(z)
     
    if scalebar:
        ax.plot([tvec[slica][-1], tvec[slica][-1]],
                [-1300, -1400], lw=2, color='k', clip_on=False)
        ax.text(tvec[slica][-1]+np.diff(T)*0.02, -1350,
                r'%g %s' % (vlimround, unit),
                color='k', rotation='vertical',
                va='center')

    ax.axis(ax.axis('tight'))
    ax.yaxis.set_ticks(yticks)
    if ylabels:
        ax.yaxis.set_ticklabels(yticklabels)
    else:
        ax.yaxis.set_ticklabels([])

    for loc, spine in ax.spines.iteritems():
        if loc in ['right', 'top']:
            spine.set_color('none')            
    ax.xaxis.set_ticks_position('bottom')
    ax.yaxis.set_ticks_position('left')
    ax.set_xlabel(r'$t$ (ms)', labelpad=0.1)
    ax.set_ylim(ylim)
    
    return vlimround