How do I generate only positive NDVI values from Google Earth Engine for my study area?

Question

Please, how do get positive values of NDVI for my study area? Please see my Javascript code below. Unfortunately, after running the code, the NDVI chart generated are mostly negative. (See image attached). How do I correct this please?

Image of generated NDVI chart

See my Javascript code:

 var startDate = '2001-01-01'
    var endDate = '2020-12-31'

var images = landsat7.filter(ee.Filter.date(startDate, endDate));
print(images);

var ndvi = function(image){
  
  var ndv = image.normalizedDifference(['B4', 'B3']);
  // for Sentinel, use var scaled = image.normalizedDifference(['SR_B4', 'SR_B3']);
  return ndv.copyProperties(image, ['system:index', 'system:time_start'])
}

var ndvi = images.map(ndvi)

print(ndvi);

var nd = ndvi.max().clip(finni);

Map.addLayer(nd, {min:0,max:1,palette:['white', 'Green']}, 'NDVI');

var chart = ui.Chart.image.seriesByRegion({
  imageCollection:ndvi,
  regions:finni,
  reducer: ee.Reducer.mean(),
  scale:30,
  
});

print(chart);